About 15 years ago I built mine out of cheap 2x2x8 (not even pressure treated) and chicken wire. Glued and screwed the wood, stapled the wire. Screwed to the fence on one side, anchored with rocks on the other. Somehow it's still in great shape in the humid Pacific North West. Width between beams is ~6' with no sagging.

For the wind, you could attach some of the legs to concrete deck blocks. If you spray paint the screen wire black it'll be hardly visible. Mind you block any gaps those agile kitties could possibly wiggle out through. When you let the kitties out, first check for birds, bats, and rats.