Someone would need to 2d scan the back (possibly the front too) in sections, something like this:
https://www.amazon.com/HP-ScanJet-4600-Flatbed-Hi-Speed/dp/B0000DIODD/ref=sr_1_3

I could photo-merge them, then trace it and create a 3d model. That plastic box is most likely vacuum formed (based on the shape, and low cost for an object that big). I would want to 3d print it in sections and glue them together to test fit before pulling the trigger on a mold.

I do have one of these scanners.

cvbell2 (resized).jpg

3d printing something that big would have to be done in pieces (my professional printer at work couldn't even handle something that big), which would be a challenge to make strong for a final part. Also thermoplastics would have better wear properties than FDM printer material. I wouldn't 3d print a mold, I would 3d print it as-is in chunks (and glue together) just to check fit. Once you know it's good, you could send that 3d model to a vendor that would CNC a mold and make as many parts as you want. Chuck Emery from spooky has both an in-house vacuum former and CNC to make the molds.

Once the original is 3d modeled, I'm sure I could make any improvements you need (within reason)

I agree with toyotaboy , and use a 3d printer to proof the design in pieces as that is large component and then get it routed out and from there could vacuum form and then machine out the holes.

as for home 3d printers the plastic is not great with heat and would warp in areas of bulbs (incandescent), so process 3 printers would be better and can alot more heat and is solid but even professional printers are not larger enough and would cost a bomb.
https://www.shapeways.com/materials/versatile-plastic

I had a congo at one stage that had this type of backbox panel at the plastic that they used will age and get brittle in time. If I were to model this I would get someone to measure from the back all the holes in a X and Y all from one corner so to be more accurate and wouldn't bother with scanning. It is a easy design once you have the dimensions.

here are some other pics to help the cause.
image_110538.jpgimage_110548.jpg

Vacuum forming is not as easy as you think. I've worked with vendors and you are constantly fighting wrinkles on corners (even on professional machines that have much higher heating elements and higher suction). I've thought about making a system, but there are 2 pinsiders on here making ramps (and spooky pinball isn't terribly far away).

Neither of those pinsiders have a vacuum forming system large enough for a backbox, I've followed both of their threads. Freeplay keeps talking about getting his larger system up, but he's so busy making smaller ramps (and dealing with life) I don't know if that's ever going to happen.

Someone up for forming these backbox pans?