Ive seen someone use a black thin plastic cable tie. Worked fine and easy to do. He lost the small (tiny) nut and did it in a pinch and left it that way.
Honestly I don't remember - I have boxes full of all sorts of bolts etc - Still have the indicators stems from my first bike - just in case I ever need them.
Done it! Lessons learnt were. A. Align the inner ring correctly. B. Gently work the out ring until it’s seated then put finger on nut and push the screwdriver hard until you can get the tread to catch on the nut. Gently tighten working the Crome ring to make sure the light is seated inside of it.
I have to add that I had a short after this resulting in no rear light. Browsing these threads indicated that the Park lights may be the cause. Tonight I undid the lights, replaced the parking bulbs, swapped out the fuses and bobs your uncle! One happy bunny here!