There was a decal sheet by Blue Rider, or perhaps Americal/Gryphon, made for LARGE 1/48th scale aircraft. It had roundels that were far too large for any kit in production. Now OOP of course but perhaps someone here still has them. I traded my set away to Brett Theobald of Roll Models or I would offer them to you. I am almost certain they would be large enough to work.
RAGIII