When cold, I use a Big Agnes Mystic long left-zip with a Ridgerest Deluxe of matching size (78 x 20). Open the bag all the way, sit on the sleeved pad in the middle of the bag, rotate about 90°, lay down, pull top of bag over you and zip up as required. You can then toss and turn inside the spacious bag (and I do) without worrying about the pad slipping around under you. IMO this setup is perfect for a Hennessey's bottom-mount system.
When warm, I use as little as possible, a cheap fleece bag or even a Coolmax sheet. It gets *very* hot and humid down here in the summer. I'm getting older and starting to wuss out on going out in the summer, sticking to spring/fall/winter camping.