Hot rocks are notorious for burning snakes.....my burrow sometimes under the substrate...I am sure yours can burrow under as well. So hot rocks are really bad news.
If you get a under the tank heater with a thermostat that is the best. Keep it on one side of the tank. I would also recommend that you purchase (when you can) a temp gun (reptile basics has these at a good price) that way you can acurately measure the heat on both sides. But for now get a thermometer with a probe. There is one called an acurite....you put it in the cage and it will measure humidity as well as temp. You put the whole thing in the enclosure with the probe under the substrate. You can get these at Lowes...or I was able to get mine at Wally World...they are about $12-. Hope this helps you out! You will get it....just takes a little fine tuning!! ;)