Yep, if you're going with magnets instead of bolting it on, you absolutely will want "Rare Earth" magnets, for the strength. Just be aware that physically they're easy to break, so be gentle with them while assembling (which can be hard when they're sticking to the side of the helmet, or each other, very strongly).
You can use all sorts of material to make your crest. Feathers could work. Leather, paper mache, and wood are all options too. The nice thing about paper mache is that if done right, you can sand it, glue it, build the magnets right into it, polyurethane it, paint it, and have the end result not obvious that it was paper mache (for that I'd use newspaper for the base structure and something soft and textured like Viva paper towels for the surface).
Historically they were most often leather or wood, (such as "
cuir bouilli", which is boiled leather and is very hard), but if you plan to make it yourself, the paper mache route might be easiest, or otherwise stitch something together with stiff leather.
