Is it a tight radius?
Inner radius or outer?
Another question, are they overhead?
Interior or exterior?
Last thing you need is someone getting clipped by a flying letter, most municipalities are now requiring mechanical attachment on overhead lettering.
If the radius is really tight, 1/4" Acrylic letters are not that bendable without some heat applied... but heating them has some risk. A panel solution might have been a better alternative.
You could make a backer with studs, then VHB the acrylic letters to the backer, then install with silicone and VHB. The letters will not follow the radius, but if the radius is not tight, it will not look bad and be safe.