Thanks for the ideas, I really appreciate your experience. Aquaseal is the consensus. Turned out I had an old tube in the freezer. A short soak in warm water and I was in business. The challenging part turned out to be clamping the joint - on a significant curve - while it cured. After trying several options that didn't get the job done, I stitched it by hand. Heavy needle, big stitches; fortunately it wasn't too difficult to get the needle through the thick rand. This drew the pieces firmly together. Now that it's cured, I'll remove as much thread as comes easily.
As for the 10-year-old deck, it's in very good shape, no evidence of anything more than cosmetic deterioration. So I think this skirt will give me a few more years of service. Quality - thanks Landis!
Zip ties - thanks for that idea, too.