Kevin,

I applaud you for your desire to build a full mask sporran. A most ambitious project. You mention that you wish to take a a pelt and glue it it a simple leather sporran you already have.

From what I have seen, structurally, full mask sporrans have a different shape from the standard three tassle type sporrans. Gluing a pelt to one is going to be a bit off looking at best.

If I may offer an alternative.

At most large craft stores one can purchase a rabbit pelt for well under $10.

I imagine that you could easily find a relatively large celtic button for under $4
(Either at the craft store or your local sewing supply / fabric store).

Now instead of trying to make a full mask sporran I suggest you make a semi-dress sporran.

Carefully remove the tassles.

Cut out the pelt to the shape of the front of the leather sporran with cutouts for the snap and the tassle holes.

Glue the pelt to to the front of the sporran.

Re-attach the tassles.

File, saw, or clip off the button back of your celtic button.

Glue it on to cover the front snap of the leather flap.

voila'

A leather and fur semi dress sporran

This will give you a chance to play with fur and leather and figure out what you are comfortable with, without breaking the bank, and with a good chance of a nice end product.

Just my two cents

Cheers
You know, I've been trying to figure out where to come up with a dress sporran before November. This might just be a viable alternative to laying out a bunch of money I don't have.