Step 1: Position the KALLAX units in a U-shape, with the opening towards the wall where you want your head to be. Any floor surface is fine, but a rug or carpet will make it less likely to slide around.
Step 2: Cut up the non slip rug pads to fit on top of the units where the plywood will go, and place them there.
Underbed storage space
Step 3: Add the plywood sheets on top.
Step 4: Cover with a bedsheet to keep clean, and put the mattress on top.
Other useful information:
You can use a single plywood sheet instead of three separate ones (we had to have it cut so it could fit in our car). Alternatively, you can also use a mattress frame/bed slats or whatever you like to sleep on.
How long and how much did it cost?
Initial assembly took about half hour. Total cost of the 3 KALLAX units, plywood, and rug pads is around $200.
What do you like most about the hack?
Simplicity and effectiveness, easy to assemble and disassemble (good for moving).
What was the hardest part about this hack?
Carrying the KALLAX units and plywood sheets up the stairs.
What to pay special attention to?
Non slip rug pads keep the plywood secure on the KALLAX base.