Make a guess at one of these https://www.ebay.co.uk/itm/233613409802 - fasten with a plastic push pin, maybe https://www.ebay.co.uk/itm/293335349638 ? Or use a fat rivet as long as it clears your bottom bracket (you can pull/knock the steel bit out of the middle of an aluminium blind rivet once it's set, and peen or file the remaining aluminium bit). Or thread the hole to use a short screw, or in a pinch use some of that double sided foam tape or Gorilla glue. I've also just used a bit of PTFE tube (like the stuff that lines cable casing) and let it find its own position, but that won't always work - it can escape when there's slack in the cable.