You've gotten some excellent advice so far from the rabble. I'd like to toss my $0.02 in just for the heck of it.

Some furs lend themselves better to a traditionally constructed sporran with the rolled gusset detail. Shorter furs like muskrat and seal are much better for a gusset roll, many North American pelts are FAR too bushy to do a good rolled edge, which is why you see some dress sporrans with a flat front (fox, beaver, skunk, raccoon).

For masks, the only advice I'd give is "Keep it sane". I've seen Black Bear sporrans, all the way down to what appeared to be a gerbil or chipmunk. A size somewhere between a muskrat and a badger/coyote is probably best. If I was going to build one for personal use I'd go with the classic Scottish "low-profile" build, where the head is structured, but not fully formed in a natural taxidermic way. No lower jaw, and with the head more flattened that it would appear in nature.

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