You are definitely wise to give yourself at least a year to work on this. Halloween costumes or even expensive professionally made cosplay items are great but nothing will ever fit you as good as something you make for yourself.
I made my Cyborg half mask out of fiberglass, but I don’t think that would work well for a full cowl as it’s 100% rigid. I know EVA foam is really popular. @TheMidnightGhost 's first video would be a good place to start. No matter how good the rest of your costume is, if the cowl doesn’t look good, it’s gonna be a bust.
I’m working on a “Three Jokers” version of Red Hood’s mask myself and I’m using a pattern I got on Etsy for about 3 bucks. Here’s something similar I found for a Batman cowl
Basically you cut out and assemble the thing out of regular cardboard or cardstock, then you can add texture with leather, fabric, etc. Or just get black EVA foam like in the video. Which seems to have a nice matte finish and it’s less bulky then texturing over a “skeleton”
Myself I’m using something I’ve never worked with before. I found some moldable plastic sheets which I’m hoping will give me flexibility and durability but we’ll see once I start putting it together.
Also a tip for masks, always remember that you’re going to have to see out of this thing. Probably at night too. The fit around your eyes has to be especially good. Red Hood has especially narrow eyes on his masks. That’s why I’m building a version that has a busted/exposed eye (post Joker crowbar to the head).
Don’t be afraid to take shortcuts either. If you can save yourself time by spending a few bucks on something ready made you probably won’t regret it. This piece seems like a good deal to me. Twenty bucks on Etsy.
Good luck!