Our own Matt Newsome (kilt maker, kilt historian, currator of thge Scottish Tartans Museum, and all round nice fellow) put up some advice advice on his Blog about sporrans:

http://blog.albanach.org/2006/07/black-v-brown.html

He has a line of hand made sporrans from Brit Fergusson, one of which is a black leather sporran with braided tassles (like the Rob Roy you pictured), but with an all leather cantle. I believe he said it would work for both formal and casual.

It might be worth taking a look or sending him a PM

Cheers

Jamie