I don't have any personal experience with heat rocks but have read about them being burn hazards for BPs several times. Apparently they don't heat evenly and are too unpredictable to be trusted. I am building a new enclosure now and have considered adding a basking shelf warmed by a small UTH on the side of the tank, you could do something similar using either a UTH or an RHP (although I didn't realize those were so spendy).