To mount the laser diode (or a pen for first use) the print head carriage needs to be modified.
I found those cover plates from a desktop computer quite nice that cover empty PCI slots. Besides, one cover just happened to lie in my line of sight. Poor thing.
Somehow I managed to bend, saw, drill and screw the plate to the carriage. Just be creative in this step and keep the precision up. Precision in building is your friend but can be your worst nemesis once you neglect it !
The carriage was not in a 90° angle to the scanner bed. Luckily a a little nut saved the day.
Before that I found a little pulley that used to hold the tape in a walkman. I installed it but realized it bumped against the x-axis rod. Had to remove it. But it is definitly worth to keep for later improvements.