I had a cool top hat (prob is still knocking around the place somewhere) but it was too small / tight for my bonce... So not to be defeated I made a tool / jig thing from a thick lump of wood, kind of "section thru head" size, then sawed it in quarters and introduced some threaded studding, washers and nuts.
This enabled me to enlarge the hat by damping it and then screwing the expanding blocks apart, stretching it to a suitable size, after leaving overnight to set obviously. Seemed to work pretty well.