Hmmm.
There's probably lots of different parts, grommets, washers etc. that would work. A well-stocked hardware store with lots of bins of the "fiddly parts" for various things would probably turn something up.
I realized that with that plastic disk, you could very easily scribe reference marks onto the disk and the flashlight host's bezel for different pre-measured focus distances. That would make impromptu burning and balloon popping demonstrations run very smoothly!