Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4902606.
As you can see in the last pic, my Glü-Bot cap failed at two points: not only did the ring break off, but the actual cap is also starting to split down the middle, so a temporary fix made no sense. I was surprised there was no available STL, so I went ahead and made one. I also added a loop to the top so you can use a string or whatever you have lying around to tie it to the body of the bot, but you can also fit it over the dimple in the center of the yellow cap. In my case, I had a scrap of Zing-It that matched the color. Total overkill, I know, but as I said, it was a scrap I wasn’t going to use for anything else and the color was the determining factor. Plus, it was nice and thin (1.75mm). However, in case you don’t have the adequate string, I made 2 other versions with a 3 & 4mm hole for the string. Plus the version with no hole. The pics are with the 2mm version, and before finding the Zing-It, I tried 2 other methods: a single interior strand from a scrap of paracord (white) and an old strap from a cell phone or something (grey)… Just food for ideas…
I printed mine in TPU, but the fit is super precise, so you might be able to get away with PLA.
Please comment on your results because I haven't printed the 3mm o 4mm version and I am curious about the viability of PLA for this.
Edit 11/07/21: It dawned on me that I could splice the Zing-It, so I went ahead and did that to eliminate the knots. Quite happy with the result :)