Here's an idea. Try silk.
Silk would probably be a great material for a top cover; it light and highly breathable. If you look at this site
where I bought my 10mm silk to make a hammock, they have widths from 36" - 108" and everything in between. The only down side is that it comes in white, but you could always dye it.
As far as a top cover goes, it doesn't have to be ripstop. It can be almost any material that is breathable.