Hey Deuce-
Awesome build.
I sealed my rims to run tubeless tires.
I covered the spoke nipple ends with squares of plastic tape, then sealed the rim with marine GOOP. If you're not familiar with it, you can find it with the glues and sealants at a hardware store. It comes in home/garden grade, household, and marine versions. If you know what a rim strip looks like on a bicycle of motorcycle, that's about what I covered on the rim with the goop. Run an internet search and you can find a how-to. It's pretty straightforward.
Sherm