I'll get pics up in a new thread later, but for now:
I had the sintra dealer cut the sintra to size. 1/2" wider than display size in width and 2.5" taller. I used two 5' strips of kiln dried hardwood, I forget the species, 5/8" thick, too make my hanging bracket for the top. I did not use a French cleat setup, because I wanted to completely isolate the screen from any imperfections in the wall. Instead, I drilled holes through the top bracket into the wall centered on studs. (This was before I mounted the bracket to the sintra. I used small screws to hold the brackets in place while drilling the holes.) I then drove 1/4" steel pins in to the studs so that the pins extended 5/8" from the wall. (The hole needs to be just tight enough that the pins are a press fit.) I then took the two bracket pieces and routed a 1/8" deep, 2" wide, groove on the face of the brackets. The brackets are then mounted to the sintra. The sintra sits in the groove, flush with the surface of the bracket. The bracket extends .5" above the top of the sintra. The Pin holes are drilled out now so that the pins can slide easily in the hole. 1/2" Styrofoam insulation is bonded to the back of the sintra both to give it some structure and to "deaden" the sintra acoustically. The back of the insulation should be flush with the back of the bracket.
Hanging the sintra is now easy, but a two person job. Pick up the sintra by the bracket and slide the bracket on to the pins. Use a couple of screws to fix the bracket to the wall. Start at the center and use a screw ONLY where a stud is and the wall already touches the bracket. In other words, if there is any waviness in the wall there will be low and high points in the wall. Don't use a screw where there is a gap between the bracket and the wall or you will introduce the wall's waviness to the screen. The pins are holding the screen up, the screws are only keeping the screen from vibrating off of the pins.
BTW, I used 2.25" wide trim; the measurements are based off of this.
You now have your screen projecting out from the wall 5/8". Mount strips of would 5/8" thick along the bottom and sides of the screen. They should be 1/4" away from the screen to give the screen room to move. This will make the strips 1.75" wide on the sides and 1.5" wide on the bottom. (The trim needs 1/4" overlap on the sides and .5" overlap on the bottom.) Wrap your trim in velvet and mount to the screen on the top and to the wood strips on the sides and bottom. I used magnets on the trim rather than nails. This allows me to easily remove & replace the trim and screen if I need too. Regardless, the trim is now holding the screen flush at the sides and bottom, but allowing the screen to move vertically. It is possible that the screen may bow out slightly in the center. Mine does about 1/4". However, even meticulously measuring test patterns, I can not discern any distortion in the picture. The bow is simply too small to have a measurable, much less visual, effect.
Mounting the foam to the screen is a critical operation. You are created a "sandwich" material that will have more rigidity than the original. Do it wrong, and you will cause a warp in the screen. Don't try and use huge pieces. For my 67" x 120" screen I used four 67"x 30" pieces. Spray the adhesive on both the screen and the foam. Using two people, slowly lower the foam directly on to the sintra. it will bond instantly, so do it right the first time. You and your partner will need to practice lowering the foam without any adhesive till you know how to work together and what to expect. Practice till you get it spot on right every time you try. Also, you need to use the adhesive on the plastic covered side of the foam. Spraying the adhesive directly on the foam will dissolve the foam.
You may want to mount magnets between the foam and the screen to use for mounting a masking system. One thing, when I painted my screen with Silver Fire, I could see faint outlines of the magnets in the paint. These outlines were invisible with a projected image, even on close inspection. With the OTS white paint I have now, the outlines are gone of course.
Hopefully this gives you an idea of how I did my screen. Pictures will have to wait just a bit...