I just put a piece of old comb in. If you are going to go mediums, I'd suggest doing a full box, but keep in mind (anyone doing this) as was discussed above, the further south you go the more bees you get in a swarm compared to northern locales. Our swarms around here are small, healthy and productive for certain, but small nonetheless. So, if you want to stay on all mediums and simply make it a brood box. A full 8 frames. If you have small swarms, I'd still suggest a large box. Bees like the space I think. You COULD use the equivalent of a double medium nuc too.
Lots of options.
Like anything else in beekeeping, what works for you is what works for you. :D
It may take a couple seasons to figure that out.