Slickest removable side to side bed platform I could come up with:
3/4" ply verticals and platform. Verticals hinged to platform. We had a 4" foam mattress. 6 ft x 4 ft.
Two sections for the bed platform:
Rear most Section - 24" x full width
Forward section - 24" x full width
Leading edge of rear section had a 1/8" x 1" x 1" angle piece running nearly full width. This angle was screwed into the bottom edge of the section and covered the face of the 3/4" edge.(see pic below) In the edge face, holes were drilled through the angle and into the plywood to receive dowels from the adjoining piece. To make the dowels, I drilled and screwed in lag bolts in part way and then cut off the heads to leave only the protruding shanks. These "dowels" were located to match with the holes drilled in the angle/ply and this is what kept everything level and equally supported. (see pic below)
The verticals were 4 ft front to back and 18" tall. I cut openings to allow access to the storage spaces around the wheel wells. The verticals were attached to the rear most section with hinges. A 1-1/2" x 1-1/2 "nailer" piece was attached to the opposite side of the vertical where the hinges were attached. This served two functions - 1. Provided a very solid attachment for the hinge, and 2. provided a solid connection where I could screw down through the platform to make the verticals stay ridged and hold the two platform pieces together. (see lower pic below - this would be looking from the rear of the van)
The result was a clear 49" wide x 18" tall storage space under the bed w easy access to the wheel well areas for storage. (accessible from the back doors or in the cabin) I used eight high quality pretty thick screws to hold the platform to the 1-1/2" nailers. The hinged verticals would fold flat against the bottom of the rear most platform piece for storage standing up against a wall. I covered the platform in gray trunk liner material.
Installing and removing the required first holding the rear platform at an angle to get it through the rear doors. Once inside it would lay horizontally so I could pivot the legs down. Then the second platform piece could be placed on the now vertical legs and secured by the thick screws.
imho - there is no place for 2x4's to be used anywhere in an interior build. It is overkill and wastes lots of space.