WRT to your screen, I would recommend stopping by Woodcraft
and picking up one these two items:beadLOCK® Tenon Joinery System
orPock'it Jig Kit
I built one screen using miter joints Gorilla Glued together with flat metal brackets on the backside. That was a freaking out-of-square disaster, so, I bought some more lumber, salvaged what I could from the old frame, and used the pocket-hole jig on the second frame. I butt-jointed the frame joints BTW.
It worked great comparatively speaking. The only caveat is that you need to clamp it very well, otherwise your joints won't be square. I had the benefit of using some old miter clamps I picked up years ago, but you need to make sure that you clamp the top and the bottom faces as well.NOTE: You'll also have to pick up some appropriately sized Kreg pocket-hole screws.
That said, I recently picked up the beadlock system and I think it may produce even better results. The joinery method seems more sound to me due to the mortise/tenon nature of the joint. As well it affords much more surface area for glue.
Otherwise, go to Hancock or JoAnn fabrics, pick up a roll of BO cloth and be on your merry way. Get that screen up and start watching movies the way they're intended to be watched.
BTW, I do recommend using Gorilla Glue. That stuff is awesome. Just heed the instructions, and don't use too much. IOW, read the directions!
To mount my screen, I used a french cleat. It's a very elegant solution. You just have to be sure to shim the the screen a bit if doesn't hang square. To make it I sawed a 3' section of 2" x 4" lengthwise on a 45 degree angle using my circle saw. I flush mounted the screws to make sure that there was no interferance with the cleat or the wall surface.