This has always bugged me, even as a kid. I used to imagine that these heros would put the bracers on as kids and then just leave them on until they grew into them, at which point, they'd never be able to take them off again.
Naturally, such things don't exist in the real world of armor. It comes from artists either not wanting to draw that level of detail, or just being ignorant about how real world armor needs to function.
I can't recall exactly which characters they were, but I've seen a few "making of" featurettes that demonstrated Hollywood-grade props where they cast it from either foam latex (which is a huge pain to work with) or flexible polyurethane foam. The process is pretty much just as Astillar described. Silicone rubber would be a nice alternative in that various formulations of it have a higher tear strength; also nice in that it won't absorb sweat and turn into a solid funk with prolonged wear (foam latex is super-guilty of this evil). It's bad in that it's an excellent thermal insulator, and it doesn't absorb sweat. So, it just pools there making it feel icky. You want to integrate a removable cotton padding in the design. I expect it could just hold with a friction fit.
If I understand correctly, the technique Nostrum is describing is basically a half-lap joint. If I was doing such a project, I would most likely go with something like this. Here's my feeble attempt at describing it via ascii art
M M M| |
the Ms on the left side lock in place with the Ws on the right side. It doesn't have to be embedded magnets either. It can be hook-and-eye, Velcro, embedded snaps, embedded 3M command-release tabs (one use only). All with varying ability to hide the visibility of the steam. If you wanted to get really fancy, you could use small partial sliding dovetails...but don't worry about that.
Another option (which I like less and less the more I think about it) could potentially be to make bracers in medium weight tooling leather. If you make a long and very careful skive-joint bonded with contact cement, the seam line is very close to invisible. Then, to put it on or take it off, you soak the thing (or your arm) in lukewarm water. Properly designed, this could let it stretch just enough to slide on and off. But yeah, there are safety issues. If you make the thing too tight, it could give you circulation problems, particularly if anything happens that results in swelling (one of which is circulation problems, creating an icky cycle). So with this technique, I'd either need to make the thing incredibly loose, or keep a pair of EMT scissors on hand so I could cut it off quickly and safely in an emergency...soo, yeah, probably wanna ignore this idea.