second thought. If those letters are already done..
Heat them in an oven to about 300F
lay them on a surface with a 18ft radius
let them set a while.
They will form to the radius.
Take ti out when it bends
let it cool.
Chemical polish if needed.
What he said. It's not as difficult as it sounds, I had a roommate who made a custom aquarium using 3/8" acrylic, our home oven, and a plywood jig. Heat one, throw it on the jig, throw another one in the oven. By the time the first one has cooled to set the next one will be ready to come out.
Charge them out the wazoo for it.
We've all been trapped in those jobs that just seem to hemorrhage money. At some point you've just got to call them out on their ridiculous crap.
EDIT: Ignore all that. If they've been this anal about the job so far there's no way you're going to get a clean enough result doing it yourself to make them happy. Try an acrylic fabricator like
Plexi-Craft and see if they can do it for you.