I asked the same question in another thread, but since then I've got my frame pretty much built. Here's what I did:
First, I bought four pieces of 1 x 4 fir from Home Depot - two 8', and two 6'. I mitered the ends at 45 degrees, so it could be assembled into a nice looking frame. Then I took some black cloth and stapled it around each unassembled frame member. Now I four 1 x 4 mitered frame pieces covered in black cloth. They look really nice.
Tonight I will be putting them together with large L brackets across the back to hold the pieces together. Then I'm going to test it for rigidity and warping. If it looks like it'll be strong enough to withstand the screen stretching, I'll take my blackout cloth and stretch it across the back and staple it into place. This will give me a 90 x 50 screen with a 4" black frame around it to absorb the DLP halo.
If the screen doesn't look like it will support the screen material without warping or bending, I'll run out out to Home Depot and buy a sheet of 1/8" particle board or MDF or 1/16 ply, cut it in half, and nail or screw it to the back of the frame, using lots of finishing nails, staples, or screws. A solid back is the ultimate in preventing warping and twisting. What you'd be left then is a sandwich consisting of the frame, then the screen material, then the solid back behind it. This will make for a fairly heavy screen, but one which should stand up to endless amounts of abuse.
Until I try the last part, I won't guarantee the results. For instance, it's possible that the plywood backing pressed against the back of the screen material will affect the look of it from the display side (either through grain showing up, or small dents in the fabric from irregularities or whatever. But in theory, this system should make a very attractive, VERY strong frame with a screen that is protected as well as a screen can be. You can hang it on a wall or carry it without worrying about something punching through your screen material.
I plan on finishing this tonight, and I'll post the results back here when I'm done, perhaps along with some pictures.