You need:
4x campingstuhl_standfuß.stl - for the feets
2x campingstuhl_halter_oben.stl - for the upper backside
1x campingstuhl_halter_oben_vorneR.stl - upper front right (when you sit in the chair)
1x campingstuhl_halter_oben_vorneL.stl - upper front left

20x M4x35/40 cotter OR screws and nuts

These connectors will certainly not fit on every chair, but at least on mine they do ;D
The rods are ~16mm.