Tubs are very hard to increase ambient temps in. The che might work but be sure it is secure if it is knocked over it could melt the tub kill the snake or start a fire. Firmly fixed is very important.
A third way that is also a bit crumby is to place the tub in a cardboard box the is larger by at least 6 inches. Open front not top. And use a human heat pad on low taped to the back wall (6 inches away) that will help to heat the air. I know a couple whom have used this type of method but it is not a good solution.
I know lots here suggest tubs to solve humidity issues but heat is critical to life of reptiles humidity is not as critical. Tanks offer safer and easier heating options in cool rooms. I would suggest this or better yet a custom plastic type enclosure as a better solution. Heat then humidity, solving humidity and sacrificing temps is a poor choice.