I was in Big Bend a couple of times in June with a 12X20 and the heat was god awful. Would like to go back sometime when it is cooler. Beautiful place and lots of photographic opportunities.
Send me a pm to firstname.lastname@example.org. We have 12X20 holders in stock but need to check your dimensions.
BTW, did you know Luc Novvitch (spelling?). He had a gallery in Marathon for a few years, the Sotel Gallery I believe it was called, but appears to have left the area.