If that was my setup, here's what I would do. If you don't need to plug an unplug the heater for moving it around purposes I would hard wire it. Very simple actually and you could use that existing box. Cut back the wire to where the insulation is not brittle anymore from the overheating, which is caused by either a loose plug connection or loose wire on the plug itself. Make hard connections with good solid twisting and firmly twisted wire nuts. The less possible loose connections, the better. This is when arcing occurs and in turn what you ended up with assuming all wire is sized properly for that load.
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SIKSUKR
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