The garolite xx works fine at 11x14 for stiffness. Should be ok for 12x20 with the 12" cross dimension. I think if you go bigger you might need to go with something else. I did make a pair of darkslides for someone restoring a 20x24 camera who had 2 holders but broken slides made of wood. For those I used .063 aluminum had had it powder coated black. He has had no problems.
I am not sure what the garolite is made of, but it is a laminate. From looking at peices I have cut and chipped before going to the water jet it looks like more of a fiber component.
Barry, PM me your address and I will mail you a small sample scrap of garolite xx I have sitting around so you can get an idea of what it is. If I can find them I have a couple scrap pieces of two different fingerstock profiles laying around also you can have.
For the light trap you could use velvet as well I suppose. I went with the felt only because one of the vendors ( I think it was McMaster Carr) had an adheisive backed version in a couple different thicknesses and it was the first one I experimented with. I also took apart an older Kodak/Grafmatic 8x10 (1950s vintage I think) holders and they difinetely used felt as well.
"Fundamentally I think we need to rediscover a non-ironic world"
I just measured septums on holders from two different manufacturers including one from Gundlach for a Korona. They both measured .070 thick but they are very old and the material looks like fish paper, so they probably were .0625 (1/16th) inch when new.
I just went out to the workshop and cut a corner off the piece of Garolite XX Black that Jim Chinn so graciously mailed to me. Thanks Jim.
The garolite cut on the bandsaw with a fine blade and on the table saw with a regular carbide woodworking blade. I then sanded it on a stationary disk sander and belt sander. It sands beautifully but makes an awful stink.
This garolite material is apparently a type of paper based phenolic. The stuff I finally ordered was also a grade XX paper based phenolic but unlike the Garolite material, this is very slick and smooth like ABS plastic. The Garolite has a textured surface very much like paper. They both saw well and sand well and stink like the dickens during sanding.