Combined elements of my last two homemade vacbed sessions - garbage bags encasing an air mattress.
Using garbage bags with the air mattress was definitely better than my second one with the air mattress and a plastic mattress cover. The seal was much tighter. Garbage bags just have more stretch than the kind of plastic used in mattress covers, which means they fill in every nook and cranny much better.
But I must say, my first one with garbage bags taped to the floor is still my favourite. It felt the most constricting. I think the seal was the tightest on that one, and also the fact that you are pinned to the floor means you are going nowhere. But with an air mattress you can still squirm and even move the air mattress around a bit.