I've not done this but I've read up so perhaps I can answer some of the questions.
How hot? It should be warm water. Something that would prevent you from getting a chill but not really hot.
On what? Everyone I've seen will put a potted plant in this the cham. The leave will help to deflect some of the spray, plus chams can get stressed out if they don't see anywhere to hide if they want.
How old? I don't think it is a question of age as much of a question of size. You do not want to put a baby or young cham in the shower, it can be too much for them. I would say a sub adult to adult (so, at least 3-4 inches snout to vent).
Just remember, you need to adjust the shower head so that it hits a wall. What you are looking for is indirect water spray, plus the added humidity. You don't want to put it right on them because that is too much. No water in the wild would come down as hard as it can out of a shower head meant for cleaning.