help needed 240v greenhouse circuit

I need advice on running circuit for portable 240 v heater for an attached greenhouse for freeze protection. The heater is rated at 240 v 4800 watts and manufacturer suggest 30 amp fuse. heater has a 6 foot cord as follows p136-49 msha 12 awg 3/c 300 V ul and the Plug is a Nema 6-15 or 6-30 250v. I have installed standard 120 volt circuits special items such as to frig, computer gear, and extensive home a\v system. I have never installed 240 v circuit.
The 250 amp circuit breaker box has 5 open slots. I will be running the circuit through 1 2x4 stud and single brick wall. (about 7 ft.)
The wires will be in a ruberized flex metal conduit.
Please suggest wire gage and breakers to use any positive advice would be welcome.
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