Step 1: African Two-Board Folding Chair
They're really comfortable, look great, and last forever.
This one is unusually small for the type, but is plenty comfortable.
As you can see from the drawing (picture 4) both boards are 12" wide and 3 feet long.
The slot is 11" from the bottom of the backboard.
The narrow part of the seat board is 4.25" wide.
For more details of construction, check out Jesse's Plank Chair