Tink, I use those here too! I also get the yellow chair forms... They seem to do pretty well, and hold up better than the others.. Able to rinse them out.. however, if you use koi clay or anything of the sort, they definitely get nasty feeling when you clean them..
If you look down the maintenance aisle, the blue furnace filters work pretty good too.. I cut them up, make several layers in a basket, put the yellow foam pads, and then the white foam pads... Then, if I really want it to clean up, I add quilt batting... Most all of it needs cleaned out a lot during the beginning of the season, later on not so much...