Buy a snugglesafe heat pad. I was sceptical at first and wasn't convinced out two little guinea pigs would use them. But they love it!!!
They are currently housed in our porch and although it's dry and draught free, I felt they were lacking some heat so I bought one of these pads.
I checked on them this evening, warmed up the pad and when I went back to give them their food, they were both sitting on it!
They looked really, snug and cosy!