Trick question really. There aren't really any "great" nano skimmers for a tank that size. Most of them are hang on back and will skim, but not to the levels an external or in sump design will. Protein skimmers really are most useful in systems with a high bioload where there is a lot of particles in the water that needs to be skimmed.
If you are doing a small tank like this for frag growth, I would keep just a barebones CUC in there, so you have the least amount of waste in the water column, and subsequently the least chance of drastic level changes. With a tank that size you already are going to be fighting against evaporation, temp and pH swings. If you could drill or plump it to a decent sized sump and get some other equipment in there, smaller tanks can make great grow out systems because they are so easy and cost effective to light.
Tank looks good though! Frag tanks are always my favorite. Coral is amazing.