Leaving the existing cover in place, and just removing the heater, would be the easiest option. It could be re-purposed for access to stowage in your new layout, after lining it with a piece of sheet metal painted black (or silver - the idea is to keep light from getting into the space behind the door and making it look obviously empty), and weatherstripped to keep the weather out. If there won't be inside stowage there, just insulate inside behind the sealed-up cover and install interior skin.
Re-skinning a section to cover the hole is an option, but imho the patch often looks worse than the door it replaced...
Like the tortoise, travelin' slow with the house on our back
2012 FC27FB "Ted Zeppelin"
2010 Tundra Crewmax Platinum "Silver Rhino"