I didnt use anything, the area was small enough they could all keep together or w/ momma to keep warm. When we moved them outta that area, we closed the vents in the room they moved to. This way there are no drafts or breezes seeing we have our ac on!(Florida and 80 degrees!) I throw towels here and there and they find them to gather on.
Even though I didnt use them, I would have chosen a heating pad on very low, under the bedding. But ONLY in a small area, so they can move off of it when they get too hot.And they will, they are pretty smart for natural instinct, as I had a few that NEVER gathered w/ the others, always strung themselves out flat on their back to keep cooler as they are/were fat and warm anyway!!!Lol.
Let us know when the temp hits 98, then you'd better get ready!