Well, I did the 10' long flat felled seam, and it turned out relatively well- if only I could stitch a straighter line... But there are no bunched spots, and the seam is the right width. I didn't have that kind of stapler available, so I just carefully pinned it and pulled them out as I went. Luckily, I only got poked a few times...

Now, I'll hem the edges and siliconize it before I sew on the tie downs...

thanks for the tips!

BM