Done. I bought a 4x4 piece of 1/4" underlayment from Lowe's for $8.97. I cut two separate pieces, one large, and one small, and joined them at the center of the crosspiece behind the cash box. That way, I didn't need to knock out the front or rear of the cabinet. I was able to bow the plywood enough to fit it in place. If you do a bottom panel, don't necessarily go by the dimensions that Clay suggests. Measure your particular cabinet first. Mine was like 21 1/4", and in fact it was still a bit loose from side to side after installing it.20190515_140514 (resized).jpg
After gluing the corner braces in, it should be fine. The wood looked a little too light, so I stained it with Minwax Early American stain.
All good enough for this project which will yield a player quality game. Better than parted out or trashed, I say.