You might check out .internal instead which was recently approved [1] for local use.
replies(7):
> As of March 7, 2025, the domain has not been standardized by the Internet Engineering Task Force (IETF), though an Internet-Draft describing the TLD has been submitted.
https://www.icann.org/en/board-activities-and-meetings/mater...
> Resolved (2024.07.29.06), the Board reserves .INTERNAL from delegation in the DNS root zone permanently to provide for its use in private-use applications.