http://kaidomain.com/Flashlight-DIY-and-Tools/Misc-Parts/S028303-20mm-Transparent-Plastic-Insulation-Gaskets-5pcs
Like these?
I’ve only had one light use this thing, and it just sits on top of the LED MCPCB, it’s held in place once everything is put together.
Edit: I see OP did mean the centering gasket after all.