I bought two of that general type. The standard shelf is in the bedroom and the hammock fleece type (works the same way, but spongier in the middle and harder to clean) in the computer room. Other than the center support, they are very similar in construction. You'll probably prefer that the lower legs just rest on the walls, not attach to it...

You might try some of the on-line cat supply places and see what they offer and design yours based on what appeals to you and compliments your decor.

One of my favorite sites for browsing (warning: adorable but EXPENSIVE! ) is AngelicalCat. They don't have window ledges but cat entertainment units designed to fit under windows that cats can sit on when they are not otherwise using it. Just visited it: they don't have the cat pirate ship any more (pout). You might have fun seeing what they build and if it gives you any ideas.