30mm Caplug for covering bolt holes in autobody and sheet metal. I got the original measurements from www.caplugs.com but you can scale it to what ever size you need. The cheapest I could find online was a pack of 25 for about $9 plus shipping. I figure printing 15 of them for $0.84 was a better way to go. 

The files are for PLA which is what I used to print them. However in my opinion TPU would be better, I just didnt have any on hand. 

I uploaded files for 1, 15, 24, 36, and 54 pcs.