If you don't ride for several hours I don't see too big of deal about the heat.
This one looks pretty neat (and not very bulky): Euro Pro Heidi Galloping Horse Boots < Galloping and Cross Country Boots < Horse Boots|Dover Saddlery.
Although, when I was using them, I was riding in the day a lot more for longer rides in lessons, but now, I just do little one hour or so schooling sessions, towards the end of the day. Fleece may be ok now, that I think of it......
Try using baby powder with the neoprene boots, that should help. I use Baby Powder with my boots.
Now I'm stuck on neoprene, or fleece, rather than what brand.