Watertight error for tube and slot

I have tried multiple forms of mating tubes to an end cap in order to create a sub assembly for a shell and tube heat exchanger; however, where the tubes meet the slot, the contact is never watertight. The diameter of the slot is the same as the outside diameter as the tube. I have created a different mate so that the tube would sit on a ridge in the slot creating two faces of contact, but still no luck. Any advice would be greatly appreciated.