It depends on what you want to do. If all you're trying to do is produce an Ed25519 signature you could use something like the Tillitis TKey. It's a product developed by one of my companies. As I've mentioned elsewhere in this thread it is open source hardware in the sense that the schematic, PCB design _and_ hardware design (FPGA configuration) are all open source. Not only that, the FPGA only has about 5000 logic cells. This makes it feasible for an individual to audit the software and the hardware it is running on to a much greater extent than any other system available for purchase. At least I'm not aware of a more open and auditable system than ours.