I have started to use one of those black plastic pond baskets also. I
hadn't thought about putting them on top of each other like a clam
shell. I just put my pump in there along with some scouring pads and
plastic sponges and then covered them all with lava rocks. I was
figuring I might get some biological filtration in the pads, sponges,
and rocks. I also wanted to use all this to block big stuff from
getting to my pump. I will think about doing the clam thing next time
I work on the pond.

Thanks,
James