We've used this sofa/bed for over a year now, and can vouch for its ease of use. It's also comfortable and straightforward to clean. Our mattress is a shikibuton, a traditional Japanese mattress, and it sits on a multiple-piece platform that folds down into a bed on three 250lb drawer slides. The shikibuton is about 2–3 inches thick, so it's comparable to a firm mattress, which is our preference.
A note on the design
A disclaimer before we get into the build: we didn't take the time to properly design the bed before starting the project. We knew we generally wanted it to serve two functions (folding into a sofa and extending out into a bed), so we had a rough idea to start and made the rest up as we went. To help make the build easier to follow, we made a 3D model to illustrate the order of operations.
Materials
We used ½" plywood for the carcass, platforms, and storage faces. Honestly, the ½" plywood had too much give and sagged slightly between the drawer slides when we sat on top, so we ended up adding additional supports underneath the bed. We suspect ¾" plywood would have avoided that issue, so we'd encourage anyone attempting this build to take that into consideration.
The overall bed dimension when pulled out is 75" x 50" (L x W), roughly a full-sized bed. In sofa position, the seat is 19" deep and 16" high. We intentionally kept the furniture height at 16" for comfortable seating.
Build process
Determining the curve
The walls of the van are curved, which requires one side of the furniture to be slightly curved to sit flush against the wall. To quickly determine the curve, we taped two carpenter's pencils together to trace the arc of the wall onto a scrap piece of wood, then used that as a template to cut the side pieces of the bed frame out of 12mm Baltic birch plywood.
Building the frame
We made a template with a cutout for the wheel well, doubled up for extra thickness since it supports the middle of the bed. We cut 3" strips as horizontal stretchers connecting the side panels and made sure the supports resting against the van wall lined up with the baton behind the walls so we could screw directly into them. Everything was connected using pocket holes and 1" screws.
Lesson learned: we regret screwing the frame into the wall batons at this stage. It threw the frame out of square. It would have been better to assemble the rest of the bed on a level surface in the garage first, then address mating inconsistencies on the van with shims.
Installing drawer slides
We cut a piece to position the slides at the right height and screwed it into the side panels, then added 3" stretchers just below the sliders for vertical support beyond just the mounting screws.
Building the platform sections
Starting from the back support, we anchored hinges for one section of the bed platform, which also serves as part of the couch backrest, using long piano hinges cut to size. We attached the main 19"-deep seating platform using pocket holes on the support.
A single slider on each end caused the platform to bow in the middle, so we added a third 250lb slider on the center support, another point in favor of using ¾" plywood in hindsight.
We extended the bed out and added the last piece of the platform with piano hinges, then traced and cut off any overlap so all the pieces lay flat in bed mode.
The backrest connection
Connecting the two backrest pieces was the trickiest part. Piano hinges didn't have enough give to flex with the back-and-forth movement. Inspired by a project where ratchet straps were used as hinges, we cut a strap to size, burned the edges to seal the fibers, and screwed it onto the backrest piece with a washer in between. It worked much better than the stiff piano hinges.
Repositioning and additional support
We removed the stretchers, cut off an inch to salvage usable storage space underneath, and re-attached them properly. We also added center supports to keep the middle of the platforms from bowing under weight; again, ¾" plywood likely would have made this unnecessary.
Storage doors and pulls
We used a Kreg jig to drill holes for the door hinges, adding a scrap piece of 1/8" plywood behind the half-inch panel so the hinge screws had something to bite into, then mounted the pull-down doors onto the base of the bed.
Front panel and hand holds
We added a strip of wood to the front face of the main platform (the part we pull to open the bed) and cut a hand hold by drilling two 1" holes and jigsawing the edges together, then secured it with L brackets.
Leather door pulls
We drilled holes in the door faces for leather pull straps, cut from a thin sheet of leather into strips.
Bed stopper
We made a simple stopper to keep the bed in place while driving: a ¾" hole drilled partway into a piece of scrap wood, a dowel glued snugly in, a screw driven from the top for extra security, and the handle shaped to finish.
Finishing touches
Once the details were complete, we laid down the cushion. We ordered a full-size Japanese shikibuton and found a fluffy full-sized duvet cover at home that fit over it perfectly. After setting it in place on the couch bed, we stuck on Velcro to help hold the futon in position; in couch mode, the futon pins against the wall and stays put.
Final result
We have a fully functioning bed and couch that converts in seconds: remove the stopper, pull it out, and flatten the futon cushion. We store the blanket and pillows in the overhead storage above the driver's seat, out of sight during the day. Although we struggled a bit through the build, we love how everything came together, and we already have plans to rebuild this with lessons learned.