I had the same problem with the Lock'N'Seal and silicone, and tried all of these methods with no success:
- GE I silicone
- Crazy glue
pipe primer and cement
- Melting the tubing and the Lock'N'Seal with a soldering iron to "weld" it together
Apparently it's the polypropylene. It has a oily texture that doesn't bond well.
Finally, I got some epoxy putty from Lowes, called Fix-it-Stick, that's the same as the marine epoxy that's sold on DrFosterSmith, and that another forum recommended. I sanded down the surfaces before epoxying them together, and it's water tight and very sturdy.
But I like your bulkhead ideas better, except that I was using 1/2" tubing and couldn't find bulkheads that fit.
How did you join the tubing to the bulkhead? I couldn't quite make out from the photos. Some sort of compression fitting?