My boy is getting old enough for a swing set. I'd love to get him a long slide. My plan is to span two trees, and put a slide on one side and a zip line on the other, with the slide obviously higher. Is it safe to just mount straight to the trees? I have some large like 5-7ft diameter trees I was planning on using. Is that safe? Any suggestions on where to snag a nice long straight slide? I checked a few places online and it seems crazy high. Is there some place that sells used ones?