Mr.John,
This is big problem getting cover off your heater.
Do this.
Cut plastic on side of vent on each side to floor.
This is nothing.
It just is small fill panel under vent,maybe 3/4" ?
Once this is gone cover will come off easy.
Good Evening,

Fred

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BTW the 91 heater cover is moulded plastic and looks like the heater
needs to be loose to remove the cover.