These are surprisingly hard to find nowadays. Multi-session CDR/DVD-R? I’m definitely not aware of anything that allows writing log-message granularity. You wouldn’t want the hacker to have a potentially large time window to flush the cache before it writes to disk.
If physical tamper proofness is not a problem you could build a little computer (say, based on a Raspberry Pi) that has only a serial connection and logs everything it receives on that to a SD card or internal flash. Your actual computer could send syslog to that. This mitigates any network-based attacks.