Some have already closed that exit, banning the storage of cash in their boxes. They want to keep your funds available for negative interest rates and possible bail-ins.
I think the difficulty would be finding enough safe deposit boxes. If I had $100M (let me dream, will you!) where could I store that physically? That much cash takes up a lot of space.
The Oatmeal had a post where he talked about how heavy his $200,000 withdrawal was (that was in 20's).
I think Adam Curry talked about buying actual gold bullion and storing that. Still takes a lot of space and a lot of cost in storage. $100M in gold is over 80,000 oz. Two tons of gold?
Yes, you're talking about 4.7 cubic feet of gold there. Buy the island and then you'll have a place to store 1 cubic foot of gold, to pay your routine expenses.