I'd have things split in half by egg crate, with live rock under the crate. Normally, frag tanks are fish-less, but if I were to do it I would have a 6 line wrasse in there to attempt to remove any issues. With one fish, as much live rock as you can fit, and not feeding corals you can most likely get away with water changes as the only means of nitrate removal...maybe a small
HOB filter or skimmer if they get too high.