Hi, I'm the OP.
Yes, I thought about one of those pancake electrical boxes. If I purposefully wanted to go deeper, I was thinking of maybe sandwiching two pieces of 3/4" azek together, routing a nice trim around the perimeter, and cutting out a 4" dia hole in the center to accomodate a typical 1"deep box. Then caulking around the whole thing against the concrete block.

The reason I want to go with a deeper box is twofold: I'm using 12AWG wire (and I think a pancake box is not deep enough by NEC), and I was also considering dropping a metal conduit down from behind this light to an outlet. This wat the outlet would be on the same switch.

Any pros/cons or other thoughts on that?
Much appreciated.