All I needed was a 3/8" drill bit, about 2" long, and with a 1/4" hex fitting on the end. You see I could not get a drill in to the gap, so I'd have to use a ratchet with a hex drive.
No problem, I have lots of long drill bits, lots of screwdriver bits, a grinder with a cutting wheel, a Propane/Butane torch (1610ºc / 2930ºf), and some brazing wire.
I chopped the drill bit short, chopped off the end of the screwdriver bit, covered the joint in flux, and cooked it a little. The brazing wire melted and slurped into the joint just like in the videos on you-tube, and suddenly I had a 2" drill bit with a hex drive. Not the prettiest joint in the world, and I doubt it would hold up to much abuse, but it was good enough to drill a hole by hand.
10 minutes later, I had the hole sorted out in the Centerboard case for the pin. And after a little playing about, I had the Centerboard mounted.
Progress at last.