The Sabot is a critical part of the launcher .
Find some PVC pipe which fits neatly inside the barrel .
Cut a 3 inch length , and cut a ‘cap’ not a bung , to glue on the bottom …..the bung needs to be incredibly neat fitting
This forms a cup which protect the bait , and when you bolt a lead weight under the cup , it becomes the sinker .
In such devices , the sabot is normally sacrificial ….in this case it wont be …..but I suggest you make half a dozen cause accidents do happen .
I dont the idea of PVC barrel etc ….not at that pressure ….. make sure you protect everything from the sun …and duct tape the shit out of everything as a precaution