I've had mixed results with memory foam pads. I have an ESP pad and find that even with fleece on the bottom, it still does not keep my horse's back as cool as 1" natural felt does.
It does conform well and helps with saddles that are a little larger then perfect but I still find that the felt does a much better job and the conforming ones can be had for under $100 on ebay
(not the ones with rubber backing or "sticky back" and other names they call them). Just simple, natural, 100% felt 1" thick.