Not acquainted with Bluebirds, but from the image, I'd fab one out of aquarium filter foam & spray adhesive...
Did the bus originate in the NE? Chances are good it's for an engine block warmer. Should ought be able to back trace the wires.
It looks more ergonomically friendly than mine, mounted low on the front bumper...
(Prolly not, just read your intro post- rear engine. D'oh!)
Those who say that it cannot be done should not interrupt the people doing it.