The Fat Daddy V3 Shorty is probably the best bet for brand name spring loaded stainless 510's, with the shortest depth. The drip well/top plate is 15.5mm in diamter, same size (and same connector for that matter) as the authentic V1.0 and 2.0 so it looks good on the flask.
It's also 2mm thick, just what it needs to be to sit flush with the top of the flask, and that 15.5mm diameter top plate also means you won't have to use a washer on the outside to deal with Kangxin's enormous 510 hole (somewhere around 14mm in diameter).
Thank You Croak for the very informative reply, it's most appreciated.
So it must be the part labeled as: V3 Short Body Set @ $4.99
Would you happen to know how stiff the inner spring is by any chance? I've emailed the Fat Daddy but he never told me about the spring stiffness as I requested.