This simple trick stabilizes iron pipe clamps and helps keep them free of glue. You’ll need a pair of 1″ diameter plumbers tees for each clamp. Drill a bit of extra clearance with a 1-1/8″ spade bit so the tees are an easy, sliding fit on the 3/4″ diameter pipe. Position a pair on each clamp and carry on with the glueup. Even if you get glue on the tees it won’t stick to the PVC.