A powder is a possibility but I'd use Zeaborb antifungal.
Zeasorb AF in between showers. Don't shower if you don't need to...and dry off with a clean towel each time. Even blow dry the area with low setting if necessary.

Tinea Versicolor is very very hard to get rid of. If you do succeed consider yourself lucky!

This is an organism that lives on your normal skin. When conditions are ripe... diabetics, showering too much or sweating too much, sharing towels, and in this case immune suppressing drugs, it will mutate into deeper layers of skin and live there. The antifungal product you choose must be able to penetrate down there to get to the organism. Things that sit on the surface are not going to cut it.

If you have sensitive skin, and use defatting soaps (detergent types like Zest or Irish Spring) for washing you expose microscopic cuts in the skin, where this fungus may enter. I would use Dove unscented and not too much of it either when showering. Don't scrub the skin, but pat it dry. And if necessary use a blow dryer on low, to dry yourself off. This can spread to your face, hair and back. It can get worse and create a cosmetic problem. Now is the time to nip it in the bud.

Using a sulfur acne soap may help in the early stages. This fungus cannot stand sulfur. Do not get one with salicylic acid however as it will inflame the damaged skin. Some people rub on Selsun shampoo or Nizoral into the area and rinse off the next day. This may work or not depending.

How you take care of your skin in between showers, is as important as the treatment.