Sending Multiple HL7 Messages to a single file (daily)

What I can do:
Get a subset of HL7 messages.
Send the messages to a RecordMap (with key fields from the HL7 message).
Save each message to a file.

How can I send all messages until a set time (e.g. 9am each day) to single file?

Scenario:
In the period 09:00 1 Jan 2022 to 09:00 2 Jan 2022 there are say 50 messages to save to file: File001.txt
In the period 09:00 2 Jan 2022 to 09:00 3 Jan 2022 there are say 40 messages to save to file: File002.txt
etc

I would prefer to append the message data to the relevant file as they go through the system (real time), so the data builds up in the respective file throughout the day.