Colin,
If you are looking to make your own from sailcloth rethink the material a bit.
Our club is in Portland Oregon and has been making their own for a couple reasons. We needed rain proof cover that would last a good long time as we have our gliders on the line from April 1st - October 3st each year.

Having tried different products we have settled on Soft Touch for the tops and Sunbrella for the bottoms. While over the years they get dirty the still keep doing the job after 7 years or more.

These materials do shrink a bit so make the wing covers 6" longer and a few inches wider.

Steve R.