Step 6: Plumb the barrel
This step proved to be a bit more complicated than I expected, mostly because I was using vacuum tubing/connectors on one side, but to connect to the barrel the only pieces I could find were for plumbing. Problem was, they were about 1/16th of an inch smaller than I wanted them to be. So after a bit of cleaning out the inside with a dremel, I was back in business.
I designed this system to be able to store rainwater, but have a spillover pipe that would take any excess into the pipe system and away from the house. This spillover pipe enters the main pipe on the opposite side of the valve (see pictures).
I cut holes in the barrel, as I was concerned that the small spigot wouldn't be able to handle any volume of water. I then attached the plumbing fixtures at the top and bottom and used some silicone caulking to seal the connection. I then measured, cut, and glued the pipes, elbows, and valve assembly.