I am leasing a horse that has higher withers and underdeveloped muscles behind his shoulders, causing “indents” there. My saddle unfortunately does not fit him very well. I don’t want to buy a new one for a horse that isn’t mine, but I don’t want to make him back sore either. Typically my saddle makes him a bit wither sore and leans to the right since his right side is less developed than his left. The saddle fitter recommended I use a diamond wool pad to lift my saddle up and add shims as necessary, but I think this is just making him more sore. I did order a western saddle pad that fills in the space behind the shoulders. My saddle feels really good with that pad and he seems less sore but for obvious reasons I can’t show with it. I thought about getting a Sedelogic pad but the “research” they claim to have done is nowhere to be found. I even emailed them and they were unable to provide me with the studies they claim they did. (That pad pictured here along with western pad). Does anyone have any ideas for a good half pad that absorbs a lot of impact but can also fill in that space behind the shoulders while not impeding shoulder movement? I’m looking for some Recs for higher withered, hard to fit horses.
Prolite pads are what I recommend.
Hilason makes a felt western pad that is around 40.00ish and you can just cut to the shape you want.
Based only on your description, I would be using shims to fill in the hollows beside the withers, and either 2 shims, or a thicker one on the less developed side.
A whole extra half pad just narrows the saddle as a whole.
If the shims in the hollows level the saddle - lowest part of the seat is in the middle - and provides enough wither clearance, then you’re good to go. Your saddle looks like the pommel is “cathedral” enough to provide ample clearance when the rest of it fits.
I can’t 100% tell from your photos but it looks like you have wither clearance at the front, but maybe not ALL the way back to the base of the wither?
If the saddle is touching his spine at all, anywhere along the length of the gullet, you can’t use this saddle. Pads will not fix it, period. They will just put more pressure there. It is tricky to see with a long wither, but if you can’t run a crop through there, it’s touching.
If there IS clearance all the way back, you could try to use a shimmable sheepskin half pad and shim the front third of the saddle, but it may throw you off balance by tipping the saddle back. If it does that, then you shim the whole thing to make it level. And yes, double shim the front pockets on the less developed side. Mattes makes a good thick one with felt shims.
I am not a fitter, but I have each of my horses fitted by a professional who has taught me a lot.
The flatness of the back relative to the curve of the saddle tree is often not given enough attention. Your saddle looks pretty curvey. You might want to post a picture of his naked back.
This is related to the point made above, about clearance all the way along his back. There must be no pressure point because the saddle curves too much.
@Amberley, when you mentioned a pad flocked w wool, what we’re you referring to? Does the Butet pad fall in that category?
I’m a big fan of the Thinline pads with either the Thinline shims or a similar foam material. I’ve had luck with ordering a 10mm yoga mat (my fitter recommended this) and cutting my own shims, my horse loved them!
If the saddle has no spine clearance throughout the gullet, no pads are going to help. Pads/shims are only helpful with small issues or rider balance! If the saddle is resting on the spine/withers, I’d reach out to your fitter and see if you could get the panels adjusted or see if you can find another saddle with thicker paneling. 
OP, assuming the general shape of your saddle is compatible with your lease horse, I have had really good luck with the gel pads (the ones with the honeycomb style holes for breathability. My horse likes it better than the shimmable pads we have tried. https://www.doversaddlery.com/horze-front-riser-gel-pad/p/X1-190659/?cpos=2&cexp=155&cst=gel%20pad&kcid=0190659&ctype=Search&cpid=res23053007657754260887277
