I'm in Australia now and can't measure my opening but you might be able to make a guess by looking at the pics. My stick boot was made by Oregon Aero who also made my seats. I can measure the opening this weekend when I get home, let me know if you need it. Whatever size it is, my opening works perfectly.
I was stationed at Seymour Johnson AFB from '83 to '87 with the 336 Aviation Maintenance Unit. I recognized the 336 Rocketeers patch and had to say "Hello."