And don't forget about the 'Chamberceiling' wall sections within Gothic Realms. These are intended to clip on top of the regular walls to add the kind of dramatic height you're looking for.

Thanks for all the suggestions. After having seen the pics in the "Missing Persons" thread and realizing that the new pieces are designed using the 2" tall GR walls, I'll just tik with that. Actually, I won't add the walls at all until MP comes out. The walls by themselves look disproportionately low, but those new arches do a great job of fixing that and making the whole whole sewer appear cramped! Excellent fix--much better than just taller walls, bravo!

If you need taller walls you could always kit-bash them as I did for my Build. All I did was take one of the walls and just doubled it up.