I had considered cutting strips of 4 ounce fiberglass, but broke down and got a roll of 9 ounce tape. Here its tacked to the inside hull with hot-melt glue.
The deck is still unglassed. Some epoxy did come through the deck, but I think I can deal with it.
It was hard to get the deck & hull to match up. Pushing the hull up to the deck was much easier than trying to pull the hull out to meet the deck. I posted a "help" message on the ConYak Builder's Forum. I mainly got a lot of complicated suggestions, but one worked: I made a 2x6" strip of aluminum (roof flashing) with a 90 degree bend at the end. I slid it into the gap, and pulled out on the hull while taping the halves together.
I had almost forgotten how greedily 9 ounce tape drinks epoxy. I also found that the size of the brush wouldn't allow me to spread the epoxy to the bitter ends of the bow & stern, but I think I can reach it later, once the hatches are cut.
After getting halfway through, I realized that I'd forgotten about gluing the the deck down at the tips. I rushed to un-tape the ends together, and woked some glue into the gap with a brush. A "whew" moment.