I'll add onto this, just got the bottom plate removed from my cabinet. I suspect the bottom plate on my cabinet was replaced in the past, looks like Liquid Nails was used to help secure it in place. What worked for me using my router and a "free" edge guide.

I took a piece of scrap 4X4, it was in rough shape so I ran through a thickness planer to get flat surfaces and square corners. My cabinet is 3/4" thick with a 1/4" grove in it, so I spaced the spiral cutter (1/4 Diablo DR75102) right at 1/2" from the face of the wood block.
w9xivqw (resized).jpg

To be able to fine tune my 1/2" spacing, I drilled 2 holes in the fixed base plate and tapered one of them, then ran a deck screw into it flush. This was the "pivot" point. The other 2 deck screws with nuts/washers on them have some wiggle room with the holes they run through. The nuts/washers are so that when those deck screws are tightened down the tapered head of the screw would shift the router base plate.
lTXCxHU (resized).jpg

The face of the block against the cabinet, I had sanded and waxed it to help it slide along better. The large block of wood gave me a fair amount of control and it wasn't too difficult to keep the wood block flush/flat against the cabinet.
FytLH1G (resized).jpg

I adjusted the depth so that the cutter was just kissing the edge of the original grove. This cutter was able to go through the base plate a single pass.
oNELRsV (resized).jpg

Now that I know what I'm doing, once staples, corner brackets and other items are cleared out, I could cut out a base in 10 to 15 minutes without much hassle.
n0VAFnX (resized).jpg

I'll follow up later on with putting in the new plate.

A bit of an update to offer up... so while this took me some time because I'm going overkill on this, a standard replacement could be done in 1 day if you have the tools on hand.

20180527_152704 (resized).jpg20180527_154506 (resized).jpg

0.5" MDF cut to size. This is my first time doing this and it boiled down hard core "measure twice, cut once", but with a lot more measuring and double checking myself over and over... and I got the length spot on the first try but the width was 0.25" too wide and I had to go back and cut a thin strip off the bottom plate.

20180527_161435 (resized).jpg

But with that, it dropped right in and was a good fit all the way around.

20180610_135313 (resized).jpg

Once I was ready to glue in, I used Liquid Nails Heavy Duty adhesive. I cut the tip of the nozzle so I'd get just over a 0.25" bead and applied it along the vertical edge of the opening, just over the step/lip the base plate will rest on. A bead along the cross brace and a few gobs in the corners. Dropped the plate in and pressed down. I then went over the bottom with more adhesive, trying to work it into any gaps or openings between the cabinet and base plate. Then with a rag, ran my finger along the inside lip to help work the adhesive in and remove the excess adhesive.

20180610_141102 (resized).jpg

Clamps, you really can't have too many on hand. Harbor Freight units here, so PBR beer budget going on.... clamped up and then let sit for 24 hours.

20180611_165555 (resized).jpg

Clamps removed, it is in there solid. I went in with a chisel and cleaned up some of the squeeze out on the inside.

I will be doing the corner wedge braces, need to cut some new ones out and paint. Cut out a few holes (speaker, vents, power supply and switch) and this is done.

What I've learned..... this wasn't as bad to do as I figured it might of been. My wood working skills mostly consist of watching a lot of YouTube videos of Frank Howarth and Matthias Wandel. Having a few choice tools is 85% of doing this: a router and block of 4X4 shouldn't be too difficult for most of us.

I butchered it up some more for ya...

ibfMLNq (resized).jpg
https://imgur.com/gallery/e3OXwNg

I'll be getting a speaker kit and will do the rest of the holes in the base plate at that time.

Router mounted to scrap wood to be used for a circle jig.
20180721_182702 (resized).jpg

Located center or base plate where speaker will go, drill hole. Using a bolt as the guide pin.
20180721_182630 (resized).jpg

Placed router in place, fired it up and lowered it until the cutter went all the way through the base, then raised it back up so that I was cutting about half way through the material.
20180721_182751 (resized).jpg

Did my first full circle cut at half way through. Then I moved about 0.5" past the first through hole I made, lowered the cutter to go all the way through and did a half circle cut, raised the cutter, moved past 0.5" again, lowered cutter and finished up my cut. I was left with two "ears" or "tags" holding the center cut out section in place. I did this so that the center pivot would remain in place while using the router.
20180721_183457 (resized).jpg

I used a small saw to use the 2 "tags" and free the center piece out.
20180721_183943 (resized).jpg

I went in with a flush trim bit to clean up the tags. Could of been sanded down as well if needed.
20180721_184726 (resized).jpg

The speaker kit I'm using came with an adapter place, I used it as a jig to locate where I needed to drill my 4 holes for the screws to hold the speaker in place.
20180721_215957 (resized).jpg

Holes drilled. I'm using machine screws with a taper head from the bottom up. Counter sunk the heads on the bottom side and will have nylon lock nuts so nothing will vibrate loose.
20180721_220237 (resized).jpg

Still a few simple things to do such as mounting up the power switch. When I first went to tackle this whole base plate project, I wasn't sure how to attack it but thinking things through a bit I was able to get great results... I'm nothing close to being a wood worker or such. If I have to do this again, no problem. Don't be afraid to give it a go!

Generally the bottom MFD is 0.500" thick, or rather has been on the ones I've done. 3.000" seems... excessive. Post up some photos of it? Unless it looks janky or is falling apart, it saving weight really a concern on a pinball machine?