It was time to install the fill hole/drain in the water ballast tank, so I took drill in hand, took a deep breath and drilled a hole in the bottom of the boat!
I had cut out the backing plate from 6mm ply, and then put it in the lathe to turn a recess to inset the fitting so it will end up just shy of the outside of the hull.
I drilled pilot holes for the screws, smeared everything up with thickened epoxy and put it all in place. I installed the three screws through the epoxy blobs and snugged them down. Then wiped up the excess and and left it to cure overnight.
Here is the result after scraping things smooth with a utility knife blade.
Next I coated the water ballast tank with graphite. I'm hoping this will provide a slick, durable surface so that the tank is easy to clean out when needed.
I modified the drain plug by drilling a hole through it and inserting a short length of brass rod. I then flattened the ends of the rod so that it can't slip out.
And here is the final result ready to go. The rod on the plug makes it easy to install and remove.