Some amount of taxidermy is generally done on full mask sporrans. I've seen two extremes. One is no taxidermy at all where the animal mask simply lies flat, limply on the front of the sporran. These have never looked right to me as part of a contemporary kilt outfit -- it looks too "rough".

On the other extreme, there is a vendor I see a lot at the Highland Games here in the southeast that has very taxidermied sporrans where the sporran mask may protrude out a good 8" or more from the wearer. Some people just love those sporrans, but I think they are a bit over the top.

A good mask sporran, in my opinion, should have enough taxidermy to give the mask a bit of shape and deminsion, but it should still lie rather flat.

We carry L&M's full line of mask sporrans, my personal favorite of which is the badger.



See:
http://giftshop.scottishtartans.org/sporran_mask.html

You mention the suitability of these sporrans for all occasions. I don't know if I would go so far as to say they are good for "ultra-casual" wear (I'm picturing someone wearing one with their instakilt and flip flops at the pool!), but they are certainly appropriate for both daywear and eveningwear.