Maybe it will be useful for you because it is only 3kb https://clipboardjs.com/
I used that script here for this tool 20000 forum users - Awesome Special Event! - #3709 by Oxy
It covers all browsers, there are no problems with permissions (R/W in CB), it copies so that it keeps the formatting, and not that each browser chooses how to format (but in your case, it doesn’t matter because you need plain text).