I presume Sofirn could be the right manufacturer to get this done. They have a lot of innovative ideas, they listen to their customers and flashlight enthusiasts like us here on BLF or TLF, they are very quick in developing something new from scratch and more or less they already have the right parts available to easily develop a nice 21700 host.

The C8S host on Aliexpress is very tempting for it can take 20mm drivers. It only lacks a minor update with the 21700 tube.

https://de.aliexpress.com/store/product/C8-C8S-host-black-flashlight/2933049_32818166776.html

Solarforce have their great L2x hosts but unfortunately, they don't seem to be interested in making a 21700 option. Simon might also be the right one to ask as he currently released updates on the M1, M2, L2 and L6.