For the frames, a biscuit joint works great and lets you leave the back open. Then just staple the fiber to the inside edges. If you don't have a biscuit joiner, you can still just cut a slice in the corner and put a biscuit (cheap at HomeD) in and glue it up. Saves time and, more importantly, weight, without adding a reflective back. Just and idea, worked for me.

Panny 3000 PJ, 118" Carada, Denon 3300, PS3, Axiom QS8, PSB 5T, B&W sub, levitating speaker wire