I have had some success with rooting cuttings but I would not be considered an expert by any means. Once I got a little itty bitty sunburn on one of my cuttings that was in full sun but noticed it soon enough to keep the damage to a minimum. I do know some people will take a papertowel roll and put around the stem.
Bottom heat is great for cuttings, I guess you would say a necessity so they would do well sitting on that bumper. It's the sun in Texas I would worry about. One thing I will use to try to keep my cuttings stable is a chunk of styrofoam on each side of the cutting. Or use a stake. Truly, I really would worry about the rock but I think, being new to this forum and lots of different ways people root, I will refrain from giving you further advice and maybe someone else will come along and offer more.
