I'm about to start construction of a two yak rack. I have most of a plan, but it isn't yet proven through implementation. The biggest thing up in the air is whether I will bury the posts in concrete or make it free standing so it can live near the trees, but move it away from there if bad weather is expected. I have the concrete, but I'm leaning toward the free standing option. The plan involves a couple 6' tall PT 4x4s about 7' apart. Both kayaks will mount on the same side and there will be a 15' "roof" made from a 2x4 frame and a tarp that can come off in the winter when the yaks move inside. I may architect it so that the roof is sitting atop the 4x4s and held on by a couple large, and removable, bolts. Also still up in the air is if I will put anything between the wood and the boat at the contact points.
The size is a sort of compromise that has to hold both a 13-15' boat for me and a Venus 11 for the boss. I'm hoping it will keep the sun off without getting too hot inside.
If I ever get it built and it doesn't suck, I'll post pics.