
First you need to decide if you're a roundhead or an alien-oval head!
I'm the latter, recent good fits for me are large (62cm) Giro and Abus helmets.
I've a square head and Giro lids, very popular with the masses, feel like my head is in a vice.
Agree with the comments here. My head is 66cm and long and narrow. That's a real challenge because, even when in the past helmets of that size were made, they often tended to be too round. In order to be allowed to ride time trials, I'm currently using one where the fit isn't great. The Giro Syntax MIPS sounds possible and I may try one, but I'll still be lucky if it fits (NB definitely the MIPs one - the regular Syntax doesn't seem to be made in XL).
When I first started to encounter helmet requirements to take part in cycling activities, it took my LBS about ten years to find something that fitted. That's no longer made.
I have encountered people with up to 68cm who posted looking for advice.
Might be a daft suggestion, but could you reshape one slightly using hot air
I don't think so. It would be bound to undermine the structure of the helmet and therefore any benefit that it might be able to offer.Might be a daft suggestion, but could you reshape one slightly using hot air, like a hairdryer? It's a polystyrene foam with a plastic shell after all. You'd have to be careful not to melt the foam, but if it was on your bonce at the time that wouldn't be an issue because you'd burn your noggin before that.