We have had really good luck with an O Ring Splicing kit.  You can buy these in metric or inch versions and they come with a tool that helps you size and cut it AND line it up when gluing. e.g.  https://www.theoringstore.com/index.php?main_page=product_info&cPath=607&products_id=18578&zenid=9ed43caca167650bcb1de3dbd43b3748 They come in a plastic box kit.  You can also find them on eBay and Amazon and I think Buna and Loctite are the best known. It is almost impossible to carry all the O ring sizes we have on our boats.  Sometimes, if an O ring is missing you have to buy several "guesses" to fit it and a kit eliminates that problem as you can size it by measuring diameter and length in place before you make it.  Looked around--this guy made a YouTube on it https://www.youtube.com/watch?v=MniGPMJb1H0  and this boater made one https://www.youtube.com/watch?v=pDb5CrayM-U&t=14s You would think that the glued splice would be a weak point but as a test have stretched these beyond recognition without having it separate at the splice.