A thin piece of wood or plastic in there could even stay in permanently, and just raise the Tower board up a little higher when using it, which might actually look nice.
I did manage to browse a website I purchase kitchen equipment from and they sell these flexible cutting boards, not quite 20x20 inches but they have a 2 pack 24x18 size which I could shorten to 20 inches on the longest dimension and only lose an inch on either side of the base platform inner cavity. They're only 1/32" or .75mm thick and I would cut it to fit snug so it wouldn't shift and remain there whenever I switch it up to the rollup boards to change the playing field decor/ aesthetics.
Again that was a very good tip Forked, thank you.












I did a similar thing with my son.Gave him tennis lessons and also did the sports thing with him,but he never took to chess.
He's now an infectious disease specialist.....Two proud parents.-