When I first purchased my enclosure, the guy at the pet store recommended that I get a heat rock but after a day or two I took it back for a refund because it wasn't keeping the enclosure warm enough (only 22 degrees). Plus after doing some research I realised that they aren't the best option because they can fail and can continue to heat and if the snake is already sitting on it, it won't move and will end up getting burnt.
I now have a light (with a cover) that is controlled with a thermastat and it's keeping the enclosure around 29 degrees. A bit more expensive, but totally worth it.