Kanger T4 is the best as they are rated at 2.6-2.8ish ohms. Vision Nanos are rated at 1.8ohm and will probably mess up your battery.
You can make a genesis style atomizer that is the size of a cartomizer and build it around 2.5ohms.
Look into the "Diver" atomizer, its not a clearomizer but it can be rebuilt and is the size of a carto and correct me if I am wrong, can be used for direct dripping.
Pretty much out of luck on the rebuildable clearomizer side of it. Unless you want your 510 to look like a club.