And with single width tartan, you'll have to seam two pieces together to make a fly plaid.
I'd encourage you to check out Matt Newsome's blog entry about his half-plaid. It looks a lot like the belted plaid, of feilidh mor, but is not attached to the kilt, and looks good with either box- or kinfe-pleated kilts. Since you'll likely have a bit of fabric left over, that might be a nice option. (I'm stitching mine up on Thursday.)
By the way, what tartan is it?