The majority of the material is a relatively stiff foam, with fabric on the outside. This was all from the suitcase, minimizing the necessary stitching. Salvaged suitcases are good sources of material for this kind of work – they’re often available, and the materials tend to be water resistant. The outer material possesses a reasonable mixture of flexibility and stiffness, and is often not too difficult to work and stitch through.
I cut the basic shape with a pocket knife and used a scalpel to score out the grooves where the folds are. It’s very stable in my mountain bike, as it was specifically designed to fit the frame. Fortunately, it also happens to kinda fit into my Crosscheck frame. The bag isn’t waterproof, but the foam should repel water, and any that gets in should drain out through the zip. It weighs ~160 grams. Every tool inside is held with elastic material, because I'm hella cool.
It works well, I'll probably make two small modifications at some point - a buckle to change the length of the strap attaching to the down tube. This would allow a secure attachment to both my bikes. I may stitch sections of inner tube to the attachment straps. This should increase the stability, even though it's already very stable.
Framebags are a good DIY project, they're expensive to buy and not too difficult to make. There are dozens of variations available, if you wanna have a crack at at making one, I'd recommend making a model out of cardboard to check it fits and everything. You can then cut this model up and use it as a template for cutting the material.