Re: Would this case fit a handmade Cajun accordion (see link).....
It looks just like the case that the Chinese made "Cajun" accordion I once owned came in. It was actually larger than the accordion and any LA made box will most likely fit, You might want to install some corner blocks for a final fit. It is not a terribly well made case, cheap Chinese manufacture. I wouldn't fly with it, but for transporting in the car etc, it should provide good protection.
Re: Would this case fit a handmade Cajun accordion (see link).....
A friend of mine (a professional player) bought the Igloo coolers (for fish bait) in LA for a couple of $ and he still uses them. I have a soft DJ vinyl LP record case with about 1 inch insulation around and the accordion fits in as if it is made for it. (I can send you a pic if you want)