6" can have a truck drive over it. Sleeved 2" would work, an even smaller. Sleeve the PVC/CPVC/Black pipe over metal gas pipe, copper pipe, wood dowels or the like. Figure what the max weight of your snake could be an add 50 pounds. Then you should be good.
I sleeve the PVC because it's easier to clean. I have had metal play ladders in the past but waste can cause rust before you see it.
Good luck!