at a machine quilters guild meeting the presenter suggested using the rolling base of an old office chair with a piece of PVC pipe or a rod of some sort, to put a bat roll onto vertically--that way you can move it and pull out the bat you need easily, etc.
as far as floating top--I float both top and bat--stitch down at top and then the sides as I advance. I use either pins or magnets to keep secure to backing next to the belly bar on my machine so no tucks. I will go ahead and load both back and top if it's a very large quilt (over 108"--my frame is 14') just because it seems like there's more 'wiggle' when rolling it.