Just to clarify, I realize that those fragile plastic monsters need full bottom support. All I meant was that you can use a solid top made of perhaps a couple of stacked pieces of plywood to provide that. You can even have a cross-shaped brace underneath, formed from pieces of 2x4 or even 2x6 attached vertically to the bottom of the lowest sheet in the stack. They would run front to back and side to side, sitting between the columns of concrete at each corner. A construction like that absolutely will not allow any flexing, and that's all that "full support" means.
You could even have the columns set 1.5 or 2 inches inside the perimeter of the top, and then run a another vertical brace all around the outside edge, wrapping around the top of the concrete blocks. This would make it simple to attach curtains (fabric or solid) to the bottom of those braces to hide the structure, but which will still be quick and easy to remove for service, etc.
I know you want to take no chances, but...do some research on this. It will work.
wednesday13
Backfromthedead
Fishman Dave
M
M1A1