Looks good to me, i would ad a sheet of plywood to the top if it were me. Make it a little bigger then the tank frame. that way the tank frame rests on a good level surface. With gorilla glue, it will expand, and if you set you stand on a level surface, garage florr if level, you could tweek the plywood with shims and gorilla glue will expand and harden to fill any minor gaps you might have.
A cheap black background idea, i did. Got limo dark ""static"" cling removeable tint. Cover the back and it is sharp, and removeable. You could use regular tint as well, but it would be harder to remove. High metalic content tint with be more mirror like.