Do you have any pictures of your horse's back? That might help with ideas.
I've used a wither pad with my gelding, when using an Australian style saddle from DownUnder. It was sent fitted for my mare, who is both thicker (the horizontal distance) and wider (the angle where the saddle meets the horse). With my gelding, a cheap wither pad helped a lot. I had a second, older Aussie style saddle which has just come back from reshaping at DownUnder - hopefully it will now fit Trooper without a pad.
One alternative, while you are trying to figure things out, is to use variously folded towels or Navajo blankets to fill in the gaps.
Here is what I used:
Wither pad: Fleece Western Wider Wither Pad - Statelinetack.com
Or Wintec Comfort Saddle Pad With Lift - Statelinetack.com
What you might consider for filling in at the middle of the saddle is this: Mayatex San Juan Solids Blanket Pony Size
I used one folded in half as an alternative to the fleece pad I linked to first. They are 24" x 24", so they only extend 12" past the backbone on either side of the back, and you can choose to use one thickness, or fold in half. I sometimes use one with a western saddle just to add a little bit of padding to my 1/2" felt pad.
Mind you, I've never tried it for that purpose...just brainstorming in public. Hope you figure it out.