[Arran]

I've known a few people whom have acquired Canadian Marconi sets with the convex oval lenses over the dial either missing or cracked, they simply replaced them with a piece of flat glass and never had and mechanical interference issues, that would include a Model 84 which is similar. $260 is a complete ripoff for a replacement glass, that sounds like a "Go away and don't bother me" price, and I would take your business elsewhere. I would check into clock restoration suppliers, but that might be a dead end. If flat glass won't work making a molded plastic lens would be a respectable replacement, I think that some "Airline" sets may have used this.
Regards
Arran