So, I was thinking ... we used to sell these things at The VHS. They were sweatbands, like what you find in baseball caps, but with some stick-um on the back of them. What you did was stick them on the front (or back) of the sweatband of your fedora, and it accounted for the size difference if the hat was just a little big on you, but no other sizes were available. They were only a few bucks, so most people were keen on the idea if they really wanted a certain hat.
These stick-in sweatbands came in different thicknesses, to account for just a small difference in size to a full size difference. They were also good at keeping the hat on your head. With the stick-in sweat, there was more friction, so the hat was somewhat less likely to zing off of your head with a gust of wind if you'd been sweating prior. So maybe people could get one of these things, and then order a larger Akubra to get the straight sided crown.
Just an idea.