There is no room for a 18 mm battery in a SF 6P. You can bore the internals, since there is enough thickness of the body.
The P60L works on a 3.6 V Li-Ion Cell, but since it falls out of regulation below 4.2 V, it will decrease its brightness since the the moment you turn it on.
In other words, the 6P / P60L / 18650 is not a working combo. The only rechargable solution for the P60L is two protected RCR123 in series, but they contains half of the energy (and thus runtime) of a single 18650.
Regards
Anthony