For outdoor piggies (those not in sheds where oil filled radiators tend to be used), snugglesafes microwaveable pads are pretty much the safest and only thing.
If you can get electric to your hutches then greenhouse heaters have been used in hutches but you would obviously need to make sure it was very safe; mount the heater and cables well out of the way of the piggies reach (as well as making sure hutch is insulated to keep the warmth in as much as possible)
I bought a £20 microwave from a supermarket and only ever use it for their heat pads
If you can get electric to your hutches then greenhouse heaters have been used in hutches but you would obviously need to make sure it was very safe; mount the heater and cables well out of the way of the piggies reach (as well as making sure hutch is insulated to keep the warmth in as much as possible)
I bought a £20 microwave from a supermarket and only ever use it for their heat pads