a contract. The school's could offer players a multi-year NIL contract (let's say, at $50K per year). If the player decides to transfer to another school, they not only forefeit the remaining years of their contract, but they cannot recieve NIL money from another school until the length of their contract has ended. Conversely, the school is obligated to the full length of the contract, even if the player doesn't end up "panning out". Schools and players could still choose to engage in shorter, one-year contracts, but the player would be taking the risk that if they were a bust, they might miss out on guaranteed money. Players would be trading "upside" in the future for security, and schools would be locking themselves in to long-term commitments, but would no longer have to worry about a player jumping ship for a better situation if he developed into a superstar.

It's not perfect, but I think it would cut down a little bit on all of the transfer-portal madness.