Nice design, but you could save a lot of space with vertical posts gusseted with plywood or angle bracing.
Wind topple is affected by width of the bottom cross elements, weight (CG) and or anchoring.

Even just two 2 x4s would have the strength to stand up to most wind you're likely to encounter. (I live on an exposed hilltop, a couple of months ago we had 40-50 mph gusts. One knocked one end of my 80 pound Great Auk loose of its cradle. No damage.

You could also complete frame one end and insert one end into the frame.
(in fact with kayaks being in the range of 15-23' you could even frame in both ends and when storing a 'yak, just insert a few feet, swing the other end in, then walk back to reach the other cross element.
For the extreme case, you have a 15' and a large double, the bars would be about 14' apart, a 23 footer would have to inserted one end about 12' before you can swing the other end in...)

Where I've been ruinously lazy is in sun protection. A roll down tarp would be a useful addition.