I added one under the cabinet above the toilet.
I have looked in the cavity of the slideing door in my Big Horn and I think I could put a bar there also if I used short toggle bolts, or add a brace inside the wall (which I think may be the best way. Will need to remove the door to install a brace for backing of teh towel rack. Darn,fe I don't need anohter project, If DW reads this I'll have to add it to my ling list.