Hi Bruce,
Welcome to the Herreshoff Registry forum.
Most canvas makers (experience does matter) will be able to measure your boom and make one pretty easily. They will use quality thread and fabric and closures.
An off the shelf cover from, say West Marine, is a common denominator product and likely ill fitting. Good Sunbrella fabric and thread, well cared for (wash with fresh water and wash the guano off regularly) will be a 20+ year investment. Take care of chafe too.
Custom fit is a good thing here. Recommend getting references from your local sailing community on a good canvas maker or one from the Bristol or Narragansett areas that would be familiar with the vessel.
Charles
