My masking system is probably more complicated than some of the others that have been described in this forum. I had a couple of special needs, that some of you may not have which means you can (and probably should) build a more simple solution.
1) The masking material needed to be acoustically transparent. All of my speakers are behind an acoustically transparent SMX screen (127" wide). My speakers are moved to the very outside edges of the screen (In the very first photo in my first post, the speakers were closer together than where they are at now). When the masking moves from Cinemascope to 16:9, it covers the speakers.
2) On either side of my screen, I have doors that give access to the area behind my false wall. I wanted this access to remain clear of obstructions, therefore I could not use masking panels that slide back and forth on rails. Instead, I had to find a solution for my side masking that would fit in a space only a few inches wide.
3) Even though we talk about "constant height" I have found that I still need to make slight adjustments to the height of my screen for several reasons:
a) I have varying degrees of pin-cushion distortion with my lens set-up and I need to over-scan the image onto the masking more when I am in stretch mode than when I am in pass-through mode.
b) Any aspect ratio greater than 2.35:1 ends up being a little bit shorter in height compared to 2.35:1 unless you use the zoom feature on your projector. The exception would be if you have a scaler that will stretch in increments other than 1/3 (of course then you technically would need an anamorphic lens that would stretch in the same increment).
c) When I was building my screen and theater, I did not yet have the projector and lens. I wasn't exactly sure what height would eventually work in my room. I also wanted to future-proof my screen in case I ever get another projector or lens that has different throw characteristics.
Because of the above, I opted for four-way masking using rollers on the sides and using black GOM as the masking material.
Details will follow...