You can use the UTH you have on that 40 gallon breeder hooked up to your 500R and put under your tub. No need to buy flexwatt.
If you can get the 15 qt size that would be best right now. Then when your snake gets bigger you can move him to the 32 qt.
If you put the UTH under 1/3 of the tub and set it up to 90-92F temperature, then it should be sufficient to bump up the ambient temps in the cold end to 80F. I wouldn't go lower than that.
Drill one line of holes around the tub about 1 inch from the top - about 1 inch apart. If you're using the Sterlite/Rubbermaid containers, you might need to put a hook through the mid-part of the lid and latch it to the side of the tub so the snake won't be able to push the middle edge up and escape.
If you have $12, the best tub, in my opinion, is the Really Useful Box. You can find this at Office Depot. The one that is good for your snake right now is the 18-quart (17 litre) size. (18"x15"x8").
http://www.officedepot.com/a/product...Quart-Plastic/
It is made of tough, durable plastic with a strong, escape-proof, lockable lid (don't use for a hatchling cornsnake/kingsnake/hognose or similar size - they are very skinny and can escape through the vents on the lid).
Hope this helps!