Judging by your estimated dimensions your tank should be a 29 gal or 37 gal. It's the same amount of floor space as a 20L just a little taller. Plenty of people keep adult corns in 20L's. General rule of thumb is the length plus width of the tank should be more than the length of the snake. I.e. 30+12=42". You said your snake is 30"-42". So that 20L should be enough.
If there is a certain dimension you are trying to obtain due to available space, you can use this chart to find the corresponding gallon equivalent. http://www.alysta.com/books/fishtank.htm
I would try ditching the red light/heat bulb. Then plug in the heat mat (uth). Depending on the ambient temp of your house you can even go without a heat source for a tiny bit of time till you can get a thermostat.
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