I have for years used some of these item. Quality is average at best, think 10 years ago Chinese. However on this page are lots of the weapon light you could really mod.
The page Leapers Leapers, Inc. - Hunting/Shooting, Sporting Goods and Security Gear
This item take p60 dropins and the body is so thick you could almost bore it for 26650 batteries. I’ve found two different styles at gun shows with the same part number. One is a thick aluminum body the other is a thin aluminum body tube with a solid single plastic sleeve bringing both done to a CR123A inner diameter Leapers Leapers, Inc. - Hunting/Shooting, Sporting Goods and Security Gear
This one is a single cell with a liner that allows 18350 batteries and P60 36mm dropins. Leapers Leapers, Inc. - Hunting/Shooting, Sporting Goods and Security Gear
This one is a P60 done by another Leapers Leapers, Inc. - Hunting/Shooting, Sporting Goods and Security Gear
This one is a P60 with a built in mount. It has a plastic liner for CR123A batteries when I removed the plastic liner 18650 would fit very loose. Probable big enough to fit a 20700 battery I don’t think 21700 would though. Unfortunately I now longer have it to verify. Leapers Leapers, Inc. - Hunting/Shooting, Sporting Goods and Security Gear
Warning their pressure switches can’t handle more than 2-3 amps. If you google search the items by their item number many of them can be found deeply discounted or on clearance sales. Also search Airsoft sites they like them too. UTG used the P60 format for many of their lights with D26 and D36 dropins. I think of the D36 as a pre C8 light. These are when the XP-E and XP-G R2’s were hot. In fact some still use Xenon bulbs. But if you want the rings and body to be one piece they sell it. If you use dropins they work.