We started by buying more sheets of 6mm ply. I didn't have easy access to a trailer for this lot, so instead bolted some 2.4m lengths of pine to my roof rack, and clamped the sheets of ply to that. It worked rather well.
Once I was happy it fit, I drilled some holes in it for ties, mixed up a batch of goop and stuck it on the boat. I repeated the process for the starboard side, then for the next plank forward, which goes all the way to just forward of bulkhead 3. The photo shows the port midships lower plank setting up. Lots of clamps, and lots of copper wire ties. This is as much as I'm comfortable gluing on in one go. I had to work faster than I like for fear of the goop going off before everything was clamped up. As a result I think I got as much goop on myself as on the boat.
The midships plank is joined to the aft plank with a simple doubler. I was originally going to scarf the plank, but figured I'd just (for once) follow instructions. It turned out nicely.