I did this with no heat or moisture:
I was planning to wet the slat and use the iron. I have a clothing iron that I clamp upside down roughly following these directions. That particular slat is boogered up slightly on one edge; possibly salvageable by sanding but I was not too worried about it so I decided to get a feel for where I might need clamp holes part way around. I didn't want to heat it, bend it and then be unable to set it aside until I had heated and bent all the way around. So I just rolled it on the concrete floor slowly, listening for the first sounds of cracking or the feel of too much resistance. I got a lot of resistance, but the bending strap kept it in a nice arc and it never started cracking. So I went on around and clamped the other end.
Should I heat it while it is in the form to help it stay bent?
The mando under construnction is roughly the size of a Weber Sweet Pea. The form is just under 8" at it's widest so that isn't an especially gentle curve at the bottom. This slat is a maple off cut from a cabinet shop. I suspect that other woods will give me more grief, but I don't know.