Think out of the box take half inch t and pop a smaller pu tube into the half inch then a quarter inch into that using Serius glue to bond it .l I have done this many times ( Serius Glue is a brand name of a glue its like a contact but it water proof and come in a silver tube which is sold by most hard ware shops) I have done this on my R O inlet pipe before and after the booster pump, 4 years still holding.Look for the pu hose on e bay they have many sizes. If they won`t fit in use a flame heat the pu hose for a short time it will shrink over the tube.Hope this all makes sense.