I am going to make a guess that you mean smoothbore by stating "fowler".
If so, the answer depends on whether one means a fowling piece, a smooth rifle or a buck and ball gun. A patchbox would be quite rare on a true fowling piece (normally having no rear sight, a lightweight round or octagon to round barrel, or a very large gauge barrel). I cannot think of an example of a pure original 1700's fowling piece with a patchbox off hand though I have seen a New England smoothy some might consider a fowling piece with a patchbox.
On a smooth rifle (rifle built gun with a smoothbore barrel when new) the odds could be quite high. If carved, it would often have a patchbox. There are dozens and dozens of guns that were probably made a smooth rifles that have patchboxes. Look in Shumway's Rifles of Colonial America or Kindig's Thoughts on the kentucky Rifle in it's Golden Age and you will find some makers whose guns are all smoothbored. That makes it seem likely some of them were smoothies from the get go.
A buck and ball gun which is more plain or intended for militia use would rarely have a patchbox. But there is one in the back of Shumway's Rifles of Colonial America Volume 2, the "Liberty or Deth" side-opening patchbox gun.
I think the question is "why?" If your gun needs a patchbox, put one on there. But most makers of smoothbores, excepting the smooth rifle makers, did not equip their smoothbores with patchboxes. Look at the existing trade guns, English and Euro fowling pieces, etc.
Herschel and many other makers are not bound by existing originals, and do some great and interesting work.
