Should be, when setting up a rack (or any type of cage) there is an element of trial and error (this is why it is recommended to have your cages set up and at the right temperatures/humidity before you get a new snake) I would start with paper (doesn't have to be paper towels, newspaper (printed or unprinted) or butchers paper are also good choices) and see where you are at temperature/humidity wise. if you find that your tubs are too dry, I would get a larger water dish and move it towards the heat tape. if that doesn't work, then resort to a moisture holding substrate such as cypress.

Just be careful when using non-paper substrates you can render the heat tape useless because heat doesn't transfer well through wood bark/shavings and heat tape only heats the tubs floor, not the air in the tub.