My old grandpa bought up a **** ton of weedeaters back in the 80s.. Had this very thing in mind. Like most of his brilliant ideas, he didn't follow through with it. He claimed to have come up with the idea himself, and likely did... No internet back then, but I'm sure others had made them before.
He was going to build and sell them for $75 or so, and then make and sell conversion kits for people to modify their own.
Gramps had some great ideas and plans... Shame he didn't go through with this one... You might be buying Eddie Brand trolling motors on store shelves today.