I ride in a Bob Marshall treeless. I love it and it's the one that works with my horse. He's hard-to-fit. He's round and mutton withered with a short back. I use a Lami Cell (neoprene) roper girth. The only saddle pad that worked is a Reinsman underpad. It's thin, about a 1/2", with a neoprene bottom. Anything thicker or stiffer and the saddle slips. But this pad is ugly. All the other Reinsmans I looked at, the Tacky Toos and other brands, are too thick and/or too stiff. Is there anything thinner that's nice looking? Could I possibly use a saddle blanket and no pad since the saddle is treeless?

Have you tried a Saddleright pad? They are not too bulky and provide excellent support. 

I haven't but I am looking into them now that you mentioned it. It appears, from the website, they are only 5/8" thick. That sounds good. 

I ride bob marshalls as well and have since the 90's, with that being said LOL, everyone has an opinion on what to do and not to do.

I used those Sport Cincha Girths, u know the ones that are preformed and made to mimic a saddles tree, i didnt like them.

Most of the pads I use are thinner, I find the thicker the more they roll.

Also depends on ur discipline, are u pleasure riding or barrel racing?

I use the wool casa zia saddle blankets with no issues.  

Now i have hada vet tell me "a saddle pad/blanket is just to keep ur saddle clean, the saddle should fit like a glove or a nice shoe, in essence the pad is only a "sock" so to speak"

Let me see if i have any pics....

I ride in a Bob Marshall, and the only pad that I like, for my gelding is a CSI. It's what works for us!