Step 5: rear part
now to the rear part of the cargobike. i took the handlebar's pivot from the old bike, welded it to a steel tube and then welded the steel tube to the rear part of the bike. the angle you see in the pic between the steel tube and the pivot was again something i saw on a pic. this was the second bad idea to do - more about it later.
EDIT: after using my trike for some time with NO angle and some discussions later (see comments below) i must say it would be better to have an angle between the steel tube and the pivot. measured at a christiania bike it should be 9.5 degrees in the direction you see in the pic - which is the opposite direction of the standard castor angle on a bike.