I couldn't see any pictures of the neck joint itself in the build diary, but from what I could see of the pictures of the back, it all looked level. Did you notice any gap in that heel area once you had first glued it on, or was it nice and level?

You could fit a neck plate and screw it on. It would spoil the look a bit, but it would certainly help hold the neck on. Ideally the neck plate would be wider than standard, so that it spreads the load over a wider area. The body wood directly under the neck is quite thin, and without glue helping to hold the neck on at least one of the sides, most of the force from the neck is taken by this thinner piece of wood. So something a bit wider would help spread the load to the thicker body wood at the sides. But a quick look on the web doesn't show much hope for an off-the-shelf option here.

You'd also need to be careful about screw length. You probably wouldn't get more than two long screws in at the front of the neck heel, the rear ones would probably go into the neck tenon, and have to be a lot shorter.