I don’t see how you could add any sort of rear girth to the Trailmaster. Just how it’s rigged is unclear due to the skirts.
“Custom fit” saddles are always problematical as they are a DIY program. Their success is dependent on the skill of the fitter, in this case the owner. This not an indictment of the owner, just a statement of fact.
Some maintain that with a correctly fitting saddle a pad is not necessary except to keep the saddle clean. This is usually a “maybe so and maybe not.” It really very much depends on the saddle and the horse.
I’ve found that the best pad available for a normal horse with a normal saddle (no conformational issues requireing a saddle with an unusual shape in the tree) is a wool blanket folded “cavalry style.” Here’s an example:
https://sreinhold.wordpress.com/2009/12/15/which-saddle-pad-is-best-is-there-a-right-answer/
This is the U.S. version. The British used a different system but got the same result. I don’t have a good picture of the British fold (in fact they had three different folds).
A major advantage of this system is that the pad is, in fact, a six layer laminate which not only give cushioning but also absorbs lateral movement. The McClellan saddle had no integral padding on the saddle but other U.S. military saddles did. It worked for all of them.
Once you get the fit right then you can select a pad and refine the fit of the combination.
G.