I just have to drop a quick note of thanks as today I finished installing the soffit in my theater (light tray is next) that I dare say is an almost exact replica of these soffits (went 14" wide instead of 16" on the soffit box due to room width.) Anyway, I followed this thread to the T as well as Big's prep instructions for MDF and it actually looks like I know what I'm doing. Amazing. No, seriously.
For anyone using sanding sealer for the first time (like me), this stuff is basically really runny, liquid super glue. I had a runaway drop of it land on my arm and thought nothing of it and then ten minutes later it's EXACTLY like when you get superglue on you. Post-shower? Yep, still there... covering at least 20 arm hairs so I'm not touching it. I'll wait it out.
On the light trays, I had HD cut me some 5.5" strips of 3/4 MDF this morning so I can get cracking after I get this mouse paint up tomorrow morning. Since the bottoms are 5.5" wide and they attach to the middle of the soffit's outer 2x2, am I correct in assuming the plywood blocking you used is a 5" by 5" square of 3/4 ply? I bought some 3/4 ply today as well but I'll rip those myself since I didn't know.
Thanks again guys... my room is finally looking like a theater and everyone who sees it will think I've taken a freaking carpentry class.