James and Steve,
I was messing around with the SIP that Frank described in the original post. I cut the plastic rope handle from the inner buckets instead of just laying them flat. I nearly threw them away, not having any use for them, but I put them aside for some unknown reason. (OK, it's because I'm a packrat and I figure I'd use them for something eventually.) A few days later it hit me...these plastic rope handles can be placed between the inner and outer bucket. This creates a nice gap between the two buckets and allows the water to fill quite nicely. The added benefit is there's nothing to throw out and nothing more to buy!
Jim