Two tin-foil boxes (the ones that are long and thin, for contaning single rolls of stuff like cellophane) stuck onto a more regular-shaped box, with bits from the City Ruins kits for details. Two little plastic cups (think shot glasses) for engines on the back, and then a coat of boltgun metal.

Midnight

 

May sound silly, but if you're looking for something at least remotetly similar in shape, you could look around for speed-/powerboat models.

The chassis of a GI Joe or other plastic toy vehicle could also serve as basis for a conversion.