Update v2 (January 31st 2024): Got feedback from iRupert that at the very top the clearance between printhead and holder was not enough. I never printed full hight before, so I missed it. Version 2 is now just a tiny bit slimmer to allow the printhead to clear the holder at the very top.
Because I couldn't use the included Bambulab A1 spool holder for the single spool as I don't have the necessary height, I created a back/side spool holder which also attaches to the top aluminium extrusion and extends to the lower back of the printer.
The spool holder consists of two parts:
To mount the spool holder to the printers frame, slide the front part over the frame, then push from the front and lower the back part at the same time.