Hi, Pic attached will help. I want to put a triangular sun shade up in the garden. I have one tree to connect a bungee cord to but 2 other points are fairly low level fence posts. I want to fix something to the fence post which can stay 'hidden' against the fence post but then when I need it, I can ping something up (to create the height needed) and attach my bungee cord/sun shade. The best way I could describe what I need is a kitchen drawer runner to slide up and lock into place and be strong enough to take the strain, then when I don't use it, I can simply slide it back down again. Any ideas? Thanks in advance.