Chiming in a bit late here. When I got my boat, the jib cleats were mounted outboard of the coaming, on the side deck, making them Very hard to manage at all. I could tell the cleats used to be mounted on the centerboard trunk from old holes.
I was worried about whether screws would hold the cleats securely, so, using Greenlake's jib cleat bracket as a guide (THANK YOU, sir!) I made my own. I got a piece of 1/4" 316 SS tube from Mcmaster-Carr, and cut it down to a U-channel that fit the centerboard trunk snugly. 1/4" was overkill, but it's what I could find to fit.
I carefully drilled holes thru the sides of the centerboard trunk and thru the bracket so that everything could be tightened down with long 1/4"-20 bolts. There is just enough space inside the centerboard trunk to accept a bolt without hitting the centerboard itself. I positioned the bracket such that it had 1/2" of space underneath it, between the bottom of the bracket and the top of the centerboard trunk, so there is room for the jib cleat washers and nuts. I shimmed under the bracket with two narrow strips of 1/2" starboard plastic, just so it doesn't wiggle at all.
I drilled holes in the bracket to mount the cleats, and then attached the bracket to the boat.
The thing is rock solid, we've sailed it all summer, no movement, and the jib cleats are such a breeze to use when mounted on the centerboard. And if I ever want to install swivel cleats, or anything else, it's just a matter of unbolting the bracket to make changes.