+1 for the Therm-a-Rest Z-lite Seat Pad   that @REI-JohnJ mentioned.   After arguing (with myself) back and forth about dropping the $40 on mine, I now take it everywhere with me.  I've sat on wet tree trunks, barely dried up mudholes, frozen rocks and just about everything else.  It doesn't actively heat the bottom but it does keep it from getting frozen or wet (be careful in the mudholes : ) ).

Superusers do not speak on behalf of REI and may have received
one or more gifts or other benefits from the co-op.