Well, I use the human heat pads for my Burm, so I think it should suffice for your snakes. I find them inexpensive, long-lasting and predictable. I wish they had the exact temps on them when heated, so you'll know if it's 70 or 170 degrees, you know? The best thing is to feel for yourself, on each 'mode'. If one is too hot, cover with substrate (Bandula, my burm, tends to uncover them... but I suppose he knows what temps are best for him) =)
Hope this helped!