I would use solid pipe either 2 or 3 inch would work. If possible can you bury a plastic drum for the line to empty into. There will be a lot of scales and they will clog up a perforated pipe. The drum will catch the scales and the overflow can run into the woods. You will also want to top it with a board or some sort to filet the fish on to keep your knife from hitting the porcelain and to keep the fish from slipping around. Good luck. I have one just like it that I haven't put to use yet. Show us some pictures when you're finish.
"gene"