that is about the only option with a tub be sure that your ambient air temps don't dip too far as well. They should not be cooler than 78ºF really you might be able to get away with 75 for a short time but not for the winter.

You might give thought to a fail safe t-stat for the primary as you are covering 100% of the bottom and a failure will be very bad as there is no escape from the heat. Tubs hold humidity well but are a pain to heat properly, limited options at least in cool rooms.