One thing I've found which works much better than a heating pad for baby squees is a seed-starting mat with thermostat.
It is waterproof so you can wipe it down with soap and water if the babies make a mess, and if you are clever enough you can probably even use it outside in a RC since most are made to be set up in outdoor greenhouses.
A germination mat is also VERY thin-- you can slide it under a shelf inside an enclosure with the cord trailing out so they can't chew on it if they aren't in a container.
They also tend to have a much more even distribution of the heating elements, which are thin and usually go edge to edge.
Seed mats also is designed not to turn off, since the point is to maintain a temperature for seed germination. This is important since these days, most commercial heating pads auto-shut off for safety reasons.
The thermostat is also great-- you can easily adjust up or down, depending on how warm your babies need to be kept and it will tell you the temp it is holding at. It is also not manufactured with the mat but plugs in and can be swapped out if you need to get a new mat, you can use the same thermostat.
They also tend to be reasonably inexpensive, all things considered.
Just thought I'd mention this since I hadn't seen it mentioned anywhere else, and it gives me MUCH more peace of mind to know I have this many safety and stopgap measures in place to make sure the babies stay warm and safe!
https://smile.amazon.com/VIVOSUN-See...s%2C152&sr=8-3