If I had a boat paddle stock that was missing the sling swivels I'd simply install studs and run pins through the holes where the swivels used to be.
Take a look at the stainless .30-06 on the right in the picture below. It came in a brown laminate stock which I replaced with a Zytel stock I purchased used. A previous owner did exactly that.
If anyone is really interested I can pull the stock and take close-up pictures of the work done inside the forearm.