There is not a lot of options out there if you want to make a cowl that fits your own head perfectly. You would need a soft elastic cowl made of urethane or latex, like all the movie cowls are made.
The best way would be to sculpt the cowl in clay on a live cast of your own head (or at least a head the same size as yours), then cast said cowl in urethane or latex (latex shrinks about 10% though, so you'd need to take that into account). If you have never done that before, but feel up to the task, I suggest you watch the smooth-on youtube channel, they have a lot of tutorials on mould making and casting, and they offer almost all the materials you need. It's not going to be cheap though.
Now if you insist going the pepakura way, the same process applies except you skip the sculpting part (and therefore cannot guarantee a perfect fit), but you'll have to resin the pep cowl to harden it, then smooth it using bond/car filler/wood filler, make a mould and cast it in urethane or latex. Probably slightly cheaper than option 1.
You can also search the junkyard section here to see if somebody offers a cowl you like that would fit your head size, if it does, the fit is usually pretty good. The average price for one of these is around 200$, cheaper than trying to make your own.
Finally, if you don't actually want a soft latex/urethane cowl, you can stick with pepakura, but apply the resin and bondo if you want to eliminate the boxy look. To get a hard cowl on, you could cut the jaw and re-attach it with elastic fabric, like Armatus Cosplay did on his custom batsuit (his thread is here :
Custom Batsuit: practical combat armor (Update 2-8 suit up pics))
Anyway, best of luck to you, and don't forget to use the search engine of this forum, you'll find a lot of build thread that might help you !
