Mounting them to aluminum won't be that great considering that on a RF amp, there are only 4 small places where the amp will contact the plate. also, the plate doesn't have a large surface area for heat transfer.

That said, those amps get extrememly hot!!! I had a 600a4 that would thermal on a very simple load (Diamond Audio S600s and a single IDQ12 d2).

If you wanted to mount them under plexi, you have a few options.
1. Cut a design out in the plexi to let air escape - pretty simple operation if you have a router
2. Use some 12v fans to circulate air in and out of the rack. This is also a pretty simple operation, but takes some planning to pull off without creating too much noise. You also have to get the air from somewhere and let it exit to somewhere....all considerations.

Go to www.bcae1.com to learn more about using fans.