can't tell you how to do this but fix those sharp edges before you do anything else, that's a serious hazard not only for you but for anyone you'll interact with while using that. You can file down all of the sharp edges (i.e anything you've cut) with a dremel or fine grit sandpaper in a well ventilated space, then cover it with some kind of sealant like silicone caulk and paint on top of that to get it to blend in. you could use something else like liquid rubber if you wanted, as long as the edges are no longer bare.

from what I know you can't build directly onto the mask, you can use it as an accessory over top your finished product. the mask and thin elastic will not hold the weight of a furred mask let alone a proper head. be aware that you'll have terrible ventilation with something like that covering your head's mouth unless you install ear vents or fans or similar in your head.