The best alternatives to OnlyKey are Opensource Builders, Fido, and YubiKey 5Ci. If these 3 options don't work for you, we've listed a few more alternatives below.
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FIDO is the embodiment of a futuristic smart lock, placing its focus on security and safety above all else. Simplistic and minimalist in its design, FIDO is complex and smart in its engineering.
The YUbikey 5Ci and the Security Key is a hardware based Two factor Authentication from Yubico. Having these two form factors, any device is protected from account takeovers.