I have a herpstat that is attached to the bottom of a 10-20g UTH with foil tape. It is not in between the tank and the uth, but rather on the outside of the uth. Which is better I really cannot say but I get pretty consistent temps with how mine is set up now. The t-stat is set to 102 degrees and this gives me a floor temperature of about 91.5 degrees with 3 layers of paper towels. With reptibark it will be tricky because they can push the stuff around and possibly lay righ on the glass. I hate to say it but you NEED another way to measure the floor temp. I would recommend the probe method with an accurite thermometer because then you can get a temperature of the glass and once that is about 95 you should be set because then the temp on the reptibark should be a few degrees cooler. (A temp gun doesn't work well on the glass, but I use one on the paper towels to get surface readings) Make sure you have a very thin layer of reptibark as the ground could become too cold.
You will NOT be able to control the husbandry of your tank without modifications unless you are lucky enough to where the location your snake will be kept is already ideal. I live in south Texas and temps and humidity aren't too terrible yet I still have over half the top covered with vinyl. I will be upgrading to a 20g tank in a month and plan on doing a foil treatment for the top as well as covering three sides with some insulating foam. Check around the husbandry forum for some tips. I have thought about tubs vs. tanks a lot and will likely switch several years down the road but for now I prefer the tank.
And about spending money you dont need...I have only had the snake a month and could already put together two different tubs, hides and all, that could house a snake. Its those dang start up costs of not knowing what the hell you are doing that get ya lol