Here is a photo of a mainsheet traveller arrangement, using a wide-mouthed trapeze ring and a few bits of string.  As someone pointed out aeons ago, grabbing the mainsheet and pulling it inboard helps upwind performance. For this system to work, you need a spinnaker sheet block on an endless loop each side, either the one that is supplied as standard by the boatyard or (recommended) a ratchet block as in the picture.

The small white cord is tied to the boom to stop the ring from slipping down and interfering with the mainsheet blocks.  The mainsheet is inserted through the trapeze ring and the other line pulled through the spinnaker block.  When not in use, it can be let go and the whole thing just dangles harmlessly suspended from the boom.