Since it's nearly impossible to match the projected image with the screen frame, in particular due to some pincushion distortion by the anamorphic lens, and the fact that the projector adds a very dim frame of about 50 pixels around the image area, I decided to add a frame to capture those errant photons. For that, black velvet is the ideal absorption material. I just needed a way to add about 4 inches of that around my spandex screen.
I decided on using some foam material, and wrap that with the velvet. A durable material presented itself in Low-Density Poly-Ethylene (LDPE) foam, as used for packaging delicate electronics and such. I found a rather large sheet, 1" thick, at a local packing supply
Next, I had to figure out how to wrap velvet about the foam. To figure out the shape of such a wrap, I made a small paper model:
Then I started on the foam, by cutting 4 inch wide strips. For that, I used a simple kitchen knife, and level as a guide for the blade, to keep it perpendicular. This wasn't ideal, but worked for the most part.
Polyethylene (PE, HDPE, LDPE) and Polypropylene (PP) are a b*tch to glue, as I learned when I built kites with Tyvek when I was a teen. Hot Glue seems to work, as does any form of heat welding, and (expensive) specialty glues. I decided on a mechanical solution, good old dove tails. First I made a template out of a plastic sheet, and then used that to make the cuts with the kitchen knife.
That got me a frame with a nice fit around my screen. The bottom was hanging down a bit, but that was a solvable problem.
Then I created a more detailed pattern for the velvet. I needed about 10 inches of velvet to wrap around the 4" x 1" foam beams, leaving about a 2" gap in the back for tensioning. Here's a PDF scan
of the pattern, printing in "actual size" should maintain the scale.
The velvet is very prone to fraying. To avoid that, I used pinking shears, where the zig-zag pattern prevents fraying. I cut 4 strips, 10 inches wide, and about 10 inches longer than the edges of my screen.
The velvet is very delicate. You can see the dents I left on one strip when I kneeled on it during cutting. This has been a learning experience for me, please try to avoid my mistakes.
Then I marked the nominal length on the strips. I did one line about 5 inches from one end, then measured the exact length of each screen edge and marked the second line. For marking, I used a chalk wheel pen; fortunately my wife is into sewing and has all of these cool tools. Then I folded the strips in half, back to back, and pinned the two marked lines together.
Next, I traced a couple of cut patterns from the original, and stapled those to the velvet. I set two stacked ends on top of each other, so 4 layers of velvet alltogether. Make sure to align the length markers of the pattern with the marked line on the fabric. Also, don't staple through the part of the fabric that will be used, only what gets cut away. The staples help to keep the slippery fabric layers aligned. Then cut around the lines, again with pinking shears.
For the next step, I made a sewing template out of a long piece of card board. This helps to keep the pattern straight, and the corner at 90 degrees.
Now the seam. This is a lot of work, as it goes all the way around the edge, and on both sides. Fold in half an inch, then fold it again, and put a pin in the center every 3 to 4 inches. Sew as close to the inner fold as possible, about 1/16" in.
Now for the actual corners. Put one end of a long edge piece of velved face-to-face onto one end of a short edge piece. Align the length markers precisely, and pin them together. Use the cardboard template to mark the sew line, notice where the pattern aligns with the length marker. Pin the seam allowance, but also leave the pins in the length line for extra stability. (slippery velvet!)
Sew along the marked sew line, then trim the seam allowance, and clip the corners. If you want to go overboard (and who here wouldn't...), you can turn the corners with thread, See "Getting sharp corners using a thread tail" on this page:
Removing the pins from the length line and turning the fabric right-side out shows how the corners will look.
Then repeat with the other three corners, but make sure at the last corner that there is no twist in the overall frame wrap!