I have an Ultimate Big Boy on mine, love it, have made rides, to CO., NM & WY on it... I have never tried a Mustang so no info. there, but can't go wrong with the Ultimate. The VTXOA board had a couple for sale a week ago.
I have a Mustang and the jury is out, as a matter of fact the first month I couldn't stand it. It's broke in a little now and it seems to feel a little better. I'm considering sending it to MeanCity to some additional cushioning. I wish I had found out about MeanCity before purchasing my seat of course the wife was wanting something softer for her which the Mustang meet that need.