Are there any ECDSA smart cards out there? It'd be pretty cool to use a smart card for Bitcoin crypto.
This is pretty much what smart cards were made for. Even if there are no native ECDSA cards, you can do it on a Javacard.
I would suggest going in this direction with this project. A smartcard is like $5, and smartcard readers are much more ubiquitous than Arduinos. You can get SIM versions that will work in your cellphone, for instance.