I'm all about making use of what I have and not letting anything waste, even if it's not necessarily the most time or cost effective. So I get where you're coming from. For a really long cut for long beams, bolting together sections of heavy duty 10' unistrut with a couple feet of overlap between each piece works pretty well as a first cut guide. You use threaded rods to connect the two parallel pieces together like a ladder. That way you can customize how wide the guide is for different size trees by using different lengths of threaded rod, and you can make up 10' rails, 18' rails or 26' rails, etc, depending on what you're working on
