Finding caps for unused coolant lines has always been a PITA, outside of the limited-size and expensive OEM parts. The go-to solution of a bolt in a hose is a lot better than a vacuum cap that will fail, but I've always had trouble finding caps that were actually meant for use with coolant, especially for anything other than the 8mm of the "cursed water cap."

However, Pegasus is now carrying dedicated coolant caps (link) in a dizzying array of sizes from 1/4" to 1-3/4" ID, all with 3-ply fabric:


I'll report back in a year or so with how reliable they are, but they are quite a bit nicer and thicker than anything else I've found.

 

Good find. I've been using Dorman coolant caps for the last couple years but I don't think they're fabric reinforced. They're basically just extra heavy duty high temp vacuum caps. Been working fine so far but I like those Pegasus ones.

https://www.summitracing.com/parts/RNB-02253