This safe nonmetallic 1-gang, 8 cubic inch, PVC switch or outlet box is great for vintage trailers because it is very shallow at only 1-1/4", and it has easy mounting flanges.  

Your wiring may enter the box from either the top or the bottom without need for drilling.

Overall box dimensions are 3-1/2" tall and 2-3/8" wide.  Tabs extend beyond.