I 'm looking at the McMaster catalog right now. I found several ways to seal a 6 in or 8inch diameter PVC pipe. A low presure economy test plug comes in a 6in ID and cost like 7.60 each. This plug will hold a 1 psi back pressure. Now there is better plugs called a lockable test plug that comes in 6 and 8 inch dia. These cost 40.25 for the 6 in and 77.53 for a 8 in dia. You can put a lock on this type.

The next type is a cleanout and test plug. This is a threaded rubber plug ring that you epoxied into the non threaded plastic pipe. the pipe threaded test plug will create a positive seal with 13 pound psi. The 6 inch dia cost around 8.00 and the 8 inch dia cost a little over 15.00 bucks [part number 256655k76]

They also make a rubber plug that will go over the out side diameter of you pvc pipe and can be clamped tight to create a water tight seal. These are also made in 6.25 to 6.5 max dia. and 8.25 to 8.5 maxium diameter. All you would need to do is tighten the stainless steel band on your pvc pipe or other material. I thought I would give you guys a few options on sealing your pvc pipes.