With that kind of hole in the neck I recommend patching it with a piece of the pelt, because I think you'll need that area to work with the mask.
Study some pix of foxes and have them available.
I have settled on a bag dimension that is about 6 inches wide at the top, 8 inches tall, and 7 1/2 inches at the widest point. I use a stiff enough leather to give it body, but remain soft enough to accomodate what you need to carry.
I use a material called Sculptamold to make the headform. It's a fancy form of papier mache that you can get cheaply at art supply stores. Just make up a batch, pack it into the mask to make a headform (wet the mask so you can stretch and work the leather into a pleasing shape).
Get a couple of glass fox eyes from VanDykes - about $5.29. The eyes make the whole thing come to life (if that's what you want.)

You might pick up a couple of glovers' needles at $0.49 - they make sewing much easier.
Once your headform starts to set up mark through the eye holes to show where to set the eyes then peel the face off of it so the headform can dry. set the eyes, then re-wet the mask and stretch it over the headform. This is a good point to sew the mask to the leather flap (that you have already prepared) and let the whole thing dry for a day or so. Then you can sew the flap to the rear panel of the bag.
Here is my most recent mask, by coincidence a fox.

Next in time was this badger. The bag comes to a point at customer's request.

Here's a raccoon I made about a year ago

And this is the third sporran I ever made.

Good luck. I'll be interested to see your results. Do think about reparing that hole first, though. Wet leather sews best. Be patient. Take your time. No rush. Let the pelt dry out completely between wettings or you run the risk of losing hair through growth of bacteria.
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