I've done a lot of thinking about it, and I've looked carefully at the old fabric, which was installed at the factory. I plan to pin both sides along the seam line, sew it, and trim as necessary. It's probably not the most efficient use of the fabric, but without a pattern (which is what I'm sure the factory had), there's no way to accurately cut to size based on the already-shrunk old fabric. Once that seam is fitted, I'll be able to drape the fabric and fit it to the rest of the fuselage. The seam goes from the very back top of the tail to the greenhouse. They used a single seam (not french fell), with a second reinforcing seam 1/4" from the first. It lays very flat and is nearly invisible after tapes and paint. You'll have to be very careful when shrinking that the seam stays centered on the tubing and the top of the fuselage.
I think that you can overlap the fabric, but I believe it has to be a 2" overlap (check this with the Ceconite manual to be sure). If you don't have a sewing machine, this might be the best way. The rest of my fabric just arrived today, and it's 95 degrees in the shade, so I'm not planning to work on it until we get a cooler day, but I will keep you informed. Have fun in Glacier, it's one of my favorite places on earth. I got to see a moose close up when I was there!!
One more consideration, there isn't any structure in the exact middle of the top of the fuselage that you could glue fabric to. There are stringers to the sides (not suitable for gluing to), and the wood formers (also not suitable), so I think for me, on the Decath, the only option is a sewn seam. On the bright side, though, the seam won't be pressing on anything so it won't show much.
