Yes you are asking too much, or rather you are asking the wrong question. English saddles aren't meant to be worn with a thick cloth or numnah - that would be like getting a pair of shoes that fit your feet, then trying to wear them with four pairs of socks.... too tight!
A properly fitted, properly flocked saddle, with a nice numnah to soak up the sweat will be comfortable for the horse all day. You don't need extra.
I have this pad Toklat English Trail Pad with Pockets. I got mine on ebay for $25 though, looked brand new. It is made of really heavy duty material on the top, and the bottom you can get wool or the coolback material.