An Attestation cert is something you mostly make up yourself, generate yourself a random UUID and "claim" it as your AAGUID (ideally publish it somewhere so ppl know it exists), and make your attestation CA and then the cert.
the issue tho, CAN a software authenticator reasonably protect its attestation public key against its users?
Before you get that part done you can only really do stuff like "credential self attestation" where you make some claims like AAGUID and just sign with the credential private key and use no x5c in there, it's basically the standard for software authenticators.
Regards
My1