Depending on the size of the kids, I would build a frame out of PVC pipe with a plywood deck to hold the upper mattress and make a mattress with notches to fit inside the legs for the bottom bed. It wouldn't take much to secure the framework to the front bench seat, and I'd be good to go. Burro
had a Rube Goldberg device where they had two aluminum tube legs under a plywood shelf that then set on the ledge under the front window... kinda spindly IMHO. I wouldn't want anyone over about 35-40 pounds on that "upper bunk."