I have a few planes that instead of a cotter-pin, I used a solid wire down the hinge line. Similar to a "piano style" hinge, using all the hinges on any particular hinge line.... On the outside area, I simply bent over an "L" that's an inch or so long, and ironed a strip of matching covering over that to secure everything. If you're careful, it will all be very concealed and nobody but you will even see it.
I've got one plane that's 19yrs old that I did that on, and even though I thought I'd never need to remove the control surfaces, I did have to take a few of them off for some repair a few years ago. That wire hinge line really helped, just a simple slit in the end where I'd secured it, and a tug or two on that wire, and my control surface was off... then repaired, and reinstalled, and again a simple strip of covering over the end "L" portion, and I hopefully won't have to take it off for 15 more years.
Take a hinge to the hobby shop, and go through the wire selection carefully, and you should fine a wire that will precisely fit into that hinge(where the cotter-key was). Don't use anything else or try to enlarge the hinge itself, but a correct size wire will work... and when you find it, it will be precise. If they have one there and you find it, you'll see what I'm talking about. Good luck with it.
PS... yes those cotter-keys are a pain. I'll never use them again.
Last edited by DGrant; 02-25-2020 at 11:08 PM.