Just have a secondary chequing account that has limited funds you can lose if something does go wrong.
I know of at least two prepaid debit cards that can receive ACH / direct deposit transactions and thus should be usable for this as well.
Both the American Express prepaid card and the Western Union prepaid card are free of any monthly fee, and have low or no ATM fee. The Western Union card features direct deposit (ACH) capability and the American Express prepaid card does as well, I believe. There was one other card provider that is free and has ACH but I can't think of the brand at the moment.
So then it is fairly easy to have a completely separate "account" just for this. And adding cash to the card (loading) is $3.74 at Walmart (up to $1,000 per load, at any register) or $5 at the convenience store (maximum $500 per load).
So, like this weekend when there was an unexpected buying opportunity ... even without having a card you simply head down to Walmart, pick up a card, register it, load it with $1,000 and when you get home use Coinbase instant verification and buy bitcoins. And if Walmart isn't convenient, any convenience store where Green Dot / Moneypak is available works.
IIRC those cards (and many other prepaid cards) are limited in that you can ACH funds in but can't ACH funds out. No technical reason it can't work both ways but the prepaid card companies want the money to only flow in (so they can rack up the fees both from the user and merchants).
