Any recommendations on a good 18" speaker for our flip top?

you could go with a modern 18" bass speaker like an emminence delta or some such thing but i would look for a JBL alnico magnet speaker ike the K-151.

I would call up Weber and see if they make a 18"

Those modern 18" eminence are designed for subs and might not even fit since the basket and magnets are so big. Plus there would be no top-end.

Otherwise varidrive is right a vintage driver (might need a recone) would be a good route.