hi
i am new too just today as read your post i too am looking for ways to keep rabbits warm, i am in county Durham uk so very cold
last winter however i used a method i found on a web site (forget which one though!) i used a flat rock and heated it up in the oven, be careful though! wrap it in a Hessian sack or old towel, put it under a sheet of cardboard and place bedding material on top, snug as a bug in a rug! just don't heat it up too much, and only put it in on a night if the temp going to drop below freezing. it stays warm all night.
previous to that everything else i tried the rabbits nibbled on! home made heat pads made with rice etc. heated up in microwave. this year i may try the rice/wheat idea again and try to find some kind of metal container flat enough to place under the bedding
this year also there are 6 new members to the litter as mam and dad both escaped mammy came back pregnant the babies are now 7 weeks old and no one wants them aw