We converted our toy hauler garage to be a room for the kids; part of that was converting the single queen bed on the HappiJac lift to be a pair of twin beds. First, here is a walk-thru of our garage:
It took 2 pieces of plywood cut to the size of the twin mattresses (approximately 38″ x 75″). They are just laid in place and not affixed to the platform at all; I worried this was going to be an issue but short of sitting with their legs hanging off, it’s never been an issue.
We pushed the plywood as far back as we could without interfering with the ability of the Happijac to go up and down.
Modifying the trim is totally up to you:
We removed the trim from the front and rear of the Happijac platform. We cut the front one into 3 pieces (38″ wide for 2 of them, and discarded the middle piece).
After placing the plywood up in place, we then added a 1×1 to each end to give the trim something to bite into and screwed the trim to it.
My father-in-law built us a ladder out of 2x4s. Each side is a 2×4 and each step was made out of another. It’s overall width is approximately the same width as the opening left over between the 2 pieces of plywood.
The mattresses are the cheapest but most comfortable that we could find from a local store.