I am going to HD today, I will get back with you and show you which aluminum wall trim I'm referring to, once you see that piece, it will be more clear. I'll go ahead and try to explain in more detail. I'll refer to the wall trim piece as "border". When using the border the actual masking edge goes on the inside of the screen, not the outside, because we want the straight edge to be within the screen, not on the outside. The same would apply for a top and bottom. The top piece would have the boarder edge where the black bars begin, as if you were looking at the movie from the middle and then visually scanning to the borders of the movie, i.e. looking up and down, and both side to side. The aluminum border edge to the panel goes on the inside as opposed to the outside because if you use the foam as the interior masking border, you will have a difficult time creating a straight edge, and if you even have semi good blacks using your projector, they will never be black enough compared to the border. Using the metal edge as the true border ( painted black as opposed to leaving unpainted, aluminum) helps just with the lines that differentiate between the moviing video, and where the black bars begin ( both for top and bottom, and for left and right). Imagine standing back and looking at your 16:9 image, you have the black bars on the top and bottom. You take your panel you've constructed, velvet black material out to mate the black bars, grey material facing screen. The one inch flat backside of the aluminum border ( unpainted ) is what will be facing the screen. You've already attached small round magnets to this side of the panel to attach to the screen. The magnets "mate" will be on the opposite side of the screen, creating a "sandwich" looking at it from the side from left to right would look like this:
1/2 inch of panel (painted black) - foam material black side facing left, grey facing right .........hotglued to - 1 inch of panel not painted because this is where the foam edge fits into the siv of the aluminum border - the magnet that you have hot glued to unpainted backside of 1 inch aluminum border - screen - magnet to hold panel in place, and to adjust as necessary.
The 1/2 inch and 1 inch designators when viewing from the side are like this. Imagine looking at the panel from a cross section, the aluminum would form what would appear to be a "hangman game diagram", this is where the foam is is inserted and hot glued to the aluminum. Once you see the border trim at HD, you'll see what I mean.
To answer your questions if I can.
The magnets are not placed on the material, they can't physically, or they will fall off after about a day. The foam material isn't rugged enough, and the foam is too poreous, the hot glue wouldn't adhere to the material. The hot glue adheres to the aluminum just fine though, you'll need three across each panel because the middle will like you said, sag a little, not because of the aluminum mind you, but when the screen pulls up taut against the magnet on the left and right, the middle needs the slight adjustment also, this is where the middle magnet is important. The middle magnet is needed to be adjust and bring the panel border straight with the video black bars from left to right, or in your instance, top to bottom.. measure the 4 or 5 inches of material and visualize what you actually need to mask, once you cut the material, your kind of stuck with the dimensions, so make it count.
The sides are supported in the same manner as the top and bottom. Except for the top, I had a manual roll up too, and for the to portion, on 2.35:1 movies, the black panel will be free standing. The bottom portion of the top panel will be fine because of the magnets used, but the top portion won't sag, but it will be tall enough that if moved off of the screen, can fall. I used paper clips the "black v shaped" ones, you can find at Office Depot or Staples. You can clamp down the material to the screen at the very top next to the casing. You will not see them during viewing when dark. I don't think you'll need them for your application, but it won't hurt to pick up some "V clips" just in case. My 4:3 panels I used didn't require them. I had a DaLite Model C 16:9 106 inch diagonal.