There used to be cool Flasks available to soak these things in. I have one that is sized exactly for HS filters - one at a time. I used to use it to soak the floor model filters.
I don't see it available anywhere now - if anyone wants to order some, let me know - I will buy a stick of PVC pipe and some test caps. I think that is all these things were anyway - you'll have to spill your own glue on them for that real cool finish, and I don't know that the brush was such a hot idea anyway.
But they used very little water - I would have to pull mine out and do a test to be sure, but it seems to me that it was only a gallon to cover a filter. And I would soak the real dirty filter in one batch, then the other four in another batch. But you might very well be able to do all five in one batch if they are not real filthy. Either way, you should only be out two gallons of water, and most filter cleaning solutions make about five gallons per.
Anybody interested?