The fund current has around $8 billion, and it keeps growing.
How about an amendment that eliminates the rainy day fund and refunds the money to taxpayers? That's something Libertarians could get behind.
It's a bad idea to put giant piles of money in government bank accounts. For one thing, it's a temptation to engage in these massive "one-time" expenditures that can raise expectations and dependency, leading to long-term growth in government programs.
Don't spend it -- end it!