I used 5 mm black plastic Scrivets with the centre screws replaced with 3 mm stainless self tappers. These are easy to remove if you need to and unlike aluminium pop rivets, will not cause dissimilar metal corrosion between the aluminium and steel. I used them on the fitting strips as well as the skirts themselves. They are also dirt cheap.

The Scrivets look like this

http://media.jetpress.com/ProductDB/...600&height=600