Do you want it to cover the whole thing or half way?
I am toying with the same problem for when I get my own Shangri-La.
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I am thinking about using a life venture mossi-net (available from Millets etc -
http://www.millets.co.uk/product/090351.html) - then taking a bathtub floor, cutting it in half and stitching the net around the outside.
This will leave the toggle on the floor to attach the fly - ill add some to the net to attach it half way up as well.
You basically end up with a the bug tent, but a half size version - plus a half floor you can use if needed.
I was inspired by a guy on backpackinglight.com who showed me his version....