Some Ikea Markus batches seem to have the backrest mounting plate at a slight angle - this results in a slightly laid back position of the backrest even in the most upright setting. This wedge adds 4° of inclination to the backrest, tilting it a bit more forward and resulting in a more upright sitting position. It gets sandwiched between the base plate and the mounting plate, with the original mounting screws passing through it. No new/longer screws are needed.

I printed it flat on the bed with a high percentage (40%) of rectilinear infill to give the print more compressive strength.