scrubby pads, not neccessarily 3M. 3Ms are the finest or tighest weave so are going to have to go on top or last in the stacking or the may clog sooner. Sams Club is the cheapest for them if youve got a memberrship. dollar store ones work well. if you were to go to Lowes or Homedepot you'll find these blue colored "cut to fit" furnace filters that work excellant, and some green ones too that are bio degradable but last.i stacked my filter as such...2 layers of green filters(wall to wall just cut them to fit) 2 layers blue then as many of the scrubbies cut into quarters and thirds as it took to fill the tank to about the bottom of my weir (spillway). for winter close down and such the mesh bags may make it easier but i found they allow channeling of water so i removed them. this is the way i did my filters i switch media between the two for the outside and the basement pond..outside is a 70 gallon stocktank and basement is a 55 gallon drum. my media has been in use into its 3rd season i have had to rinse it or clean it in any way and still performs fantastic.. i can only suggest what ive done and works for me.
floor scrubber pads should be a good way to go also....especially in a barrel.