Urgent Raspberry Pi Hardening

I am sitting in a hotel with up to three Raspberry Pis on a wired link, sharing the same physical network as known attackers. One board is in normal day-to-day use, another runs Pi-hole with AdGuard; the third may or may not be powered on, but plan for it anyway... (Budget: $30 - $250 USD, Jobs: Linux, Network Administration, Network Security, Penetration Testing, Raspberry Pi, System Admin, System Administration, Ubuntu)