My phone will require a passcode if my fingerprint doesn't register enough times in a row. Maybe you can get similar functionality to what you want by just only registering one of your thumbs or something? I can't find anyway to turn the number of allowed failures down, though..
That's a decent alternative, but it's got two problems that make it a worse option:
First, it's hard to do without looking, like if it's in your pocket. Also, if someone is watching you, or trying to compel you to unlock your phone, they'll have a chance to stop you if they're paying attention.
Second, that doesn't actually work for me on my version of Android. xD