I used a spray rubber coating, designed for gutter repair, etc, to make the entire box watertight.
It's also black to absorb heat.
To finish the roof, I found some individual cedar roof tiles for sale at the local lumber yard. (I took this picture prior to painting the box.)
First I beveled them using my belt sander (careful, they are fragile.)
Finally, I brad nailed the cedar roof on top of the box.
Not pictured, now simply lower the bat box onto the post, and drive 2 wood screws through to attach it to the spacers, using the markings you made in pencil previously to eliminate guesswork (I had screws at 4" up from the bottom edge on one side, and 22" up on the other.)
Double-check that your spacer blocks don't block the direct path to the cut-outs, where the bats will scramble up to their new dwelling.
Pre-drill these final 2 holes to prevent splitting, and finish with a dab of roof caulking to ensure 100% waterproof interior.