Melt in some holes, set up the tub just as if it contained a snake (substrate, full water dish, hides, heat, etc.). Let the tub sit for a number of hours or even a day and check your temps and humidity. Adjust as required by adding more holes on the top, if you use a lid, or the sides or both. It's best to get this sorted out and stabilized before putting your new snake in it's new tub.