Actually there are a lot of opportunities during antelope hunts to shoot prone where sitting isn't an option. There have been numerous times where I've belly crawled over a ridge with nothing but a yucca plant or single sagebrush for concealment, then peeked around the cover and fired. Sometimes there wasn't even any intervening brush, just crawl until the rifle barrel clears the ridge, and fire.
The extra movement and increased silhouette to gain a sitting position has caused many an antelope hunter to be busted. Doubly so if they were crawling on hands and knees to begin with. If there's any possible chance those antelope will see you over the intervening concealment during a stalk, drag that belly through the cactus.
This reminds me if someone already didn't mention it, get yourself a pair of knee pads and some tough leather gloves for the "stalk."
I like a long day pack and stuff it with clothes I don't need so it gets the right consistency. If you use the right pack it'll have two different thicknesses (width and depth) to work with, and height too if you really need a tall rest.
Also, a light rear bag stuffed with plastic beads.
Good luck!
