How can I package a relay (for lighting control)?
Someone already did all the work for you

Fortunately, you aren't the first person to run into this problem.  In fact, there have been enough people who have had this problem that the good folks at Functional Devices slap an appropriate relay into an appropriate box, dress it up slightly, get it UL listed, and sell it!

In particular, you want something like a RIB2401SB for your job -- these are SPST-N/O relays rated for ballast loads up to 20A@277VAC, with a manual override switch and a multi-voltage coil that can run off 24VAC/DC or 120VAC. (A 208-277VAC version is also available.)