The Varitube 14mm Low Profile won't fit without a fair amount of work. It's too small in diameter for the diameter of the hole in the Kangxin top plate and it's borderline too long, risks impacting the positive lead for the battery right underneath it and finally, since it's low profile, it's about half as thick as it needs to be to sit flush, ideally you'd need to use a 1mm thick washer with a 10mm ID underneath it (on the outside, not inside the mod) to raise it up higher (which might give you about 1mm more battery clearance).
Also keep in mind that the hole cut in the top plate is not round, it's flat on one side as an index for the flat side of the original 510 connector, you'll need to do a little trimming to make it round-ish again, no matter what 510 you go with.
The Fat Daddy V3 Shorty 15.5mm is about the only one that will fit and should also barely clear the battery terminals. The big thing is it's larger in diameter which should make for an overall better fit, and the 2mm thickness of the
juice well/top plate is the same as the Kangxin, so it will fit flush. Plus, it was what Vaporflask used on the real V1 and V2 models.