Getting hung up on building a platform for a box

[AcidikBass20]

Here is what I am working with. I want to build a platform on top of this to bolt my subs to, but I can not think of a brilliant idea to get this platform to lay flat.
http://i364.photobucket.com/albums/oo81/AcidikBass20/DSC01106.jpg
http://i364.photobucket.com/albums/oo81/AcidikBass20/P1010052.jpg

I am getting a piece of 22" by 53" to play across there, and need to make sure if is secure to the frame, sturdy, and level. I think thinking either that harding foam of some pieces of aluminum bar with a hole in the middle for a bolt. Ideas?

[grimreper912003]

why not just wall it then?

[AcidikBass20]

No wall yet. Getting new subs later and might do a wall then. Just need a solid platform for now.

[AcidikBass20]

What about foam?