If you want to stay with Surefire, you can buy a 6P / 6PD and have it bored out to accept the 18650 battery. You'll want to have the 6P tube bored out to 18.4mm so your light will accept most brands of 18650 batteries. There will be *plenty* of meat left in the tube so that you're not impacting its structural integrity by much if done properly on a lathe. I would not recommend grinding out the tube yourself or sanding it out with sand paper wrapped around a wooden dowel, etc. I know some people have done it, but none of these methods will give you the precision uniform factory looking results that you can achieve with a lathe.
Too maintain the versatility of your 6P so that it can still use the factory P60 lamps and many other modules that require the use of a large negative contact spring, the 6P tube should only be bored out at 18.4mm up to about ~77mm from the end of the tail cap threads. This should leave about ~4mm of the original bore left towards the front of the light so the negative contact spring (the large spring) has plenty of material to rest against. This is yet another reason why a precision tool such as a lathe should be used instead of grinding with a Dremel tool or sanding with a wooden dowel, etc. I've read where other members here on CPF and other forums have paid ~$15 to have their 6P tubes bored, so this option is very cost effective compared to buying an aftermarket tube.
I'm running the Dereelight multi-mode modules in our 6PD's which are bored out for the 18650 and this makes a really nice, powerful and versatile light with excellent battery life. I just wish the the low level was lower, but I have this same wish on all P60 modules I've tried. It sure is nice to have a choice when it comes to brightness and that's why I like the multi-mode modules.
Dereelight really makes a very nice product with excellent support from them through CPF. It would be less expensive to buy their CL1H forward clicky light which is already setup for the 18650, but I can understand why you'd want to buy USA made when possible.