Step 1: Buy a bunch of spot bitcoin and get 6-10% APY on it as a demand deposit at a regulated exchange
Step 2 (optional): Short an equal amount of CME bitcoin futures and get paid 15% APY in contango.
That's a 25% return on a delta neutral positon, just for taking some counterparty risk with the exchange and some liquidation risk. If bitcoin spikes up enough to squeeze you on the short futures position, then you cover some of the futures and sell some of the bitcoin at the same time, and wire the proceeds from the bitcoin exchange to the broker, which takes an hour or less during business hours. But I guess if too many people do that then the spread blows up whenever bitcoin spikes so it's no longer a safe arbitrage unless you have deep pockets to cover the collateral on the futures.
Seen this before (most recent in a series, first is here). Worth reading for a better understanding of the risks involved (not saying the OP doesn't understand them of course).
GAT