But even for 21mm tank, you'll still have to widen your VTR ring.
IMHO: Dual coil RTA's (Ithaka, Aqua, Fogger, Orchid, Billow) are NOT the best choice for Innokin iTaste VTR because, unless you want to deal with ridiculous flimsy 32 gauge coils, it is hard to get decent performance at 15 Watts.