I think that you should definitely "put a half round 'top collar' above the central roundel." The overall look is good, but it's a little bland without some extra show.

What about tassels? That's a niggling concern for me and the tassels are a big deal on hunting sporrans to me.

Finally, how will you finish off the top? You could use a metal cantel or just a leather finish of some sort. I like both; the metal adds a bit of the formal look, but the leather makes it just plain cool.