Maybe you can create a ‘bended plane’ that aligns with arches and try pinning lots of points at the borders of fabric?
Author of the plugin @Anton_S will probably have a better idea/comment on this.
Edit: Made a test, not bad i think. If you create more fabric (wavy plane maybe?) between arches, it’ll be better i believe.
Here is the .skp file: drape_arch.skp (990.5 KB)
