Not that I have done this- mind you.... but I have had a couple Gottlieb cabinets apart a bit and they do usually have play between the cabinet bottom and the cut out/slot it sits in- although I admit that’s usually not as prevalent up front where you need the gap-

But, I would try to wedge and even using a rubber mallet- hammer one edge in and then bend the metal into an arc and flatten with force to wedge the other side in. Gotta be a little careful- you can split stuff if it’s to tight. May be a lost cause depending on how far apart you had this and how much clamping and gluing you have done. Anyhow- obviously not an important piece for home use... hopefully that’s the destination else some work may be required.

Edit- the more I think about this the more risky it seems. I would have to look really carefully at how much I needed to bend the metal, how much gap there was etc etc etc... it would be a hell of a lot easier with the cabinet upside down... and if it’s even looking possible, whatever you do- think it through and go slow. This is one of those things I would probably try and if I broke something I would be pissed but feel like I knew there was a chance- ya win some ya loose some.

Take the metal panel, cut some off of the edges that have to go into the slots for the bottom board. Trim enough to allow one side to slip into the slot. Then slide the panel into the other side. (Like a sliding window, and how they are held in the window frame). The trick will be to have enough metal remaining to hold it in place. You could also put a few dabs of an adhesive on the final positioning.
Until the cabinet is taken apart in 20 or 30 years from now, know one will notice the smaller piece of metal.