Big Litter Pan with No Top?

Question
We have a rather large cat who has to duck to enter a covered litter pan (even a large covered pan) and a smaller cat who easily fits into any litter pan but who sometimes pees over the edge of the uncovered pans and rather widely rakes the litter so it flies all over the place if we don't use a covered litter pan. We were willing to let our larger 18-pound cat squeeze into the covered pans until recent events (some out of the box peeing and pooping which may or may not be connected to the size of the litter pan) led our vet to inform us he suggests NEVER using a covered pan. Instead, he recommended we make or purchase an uncovered pan with extra high walls. Can you tell me where I can buy one of these or exactly how I can make one? And do you agree with his position (as covered pans are readily sold in every pet store I've seen)? Thanks for whatever info/insights you can provide!

Answer
I went to Wal-Mart and got a large plastic storage box. I made sure that it is deep and rather large. I filled it three inches high with litter and voila! a large, high walled, litter pan.
I made sure to get a really strong, heavy duty, storage box from Rubber Maid or something equivalent. It works very well indeed.
Good luck,

Barb