what exact is the problem with the certification of box fixture with the truck ?

As long it follows the builders manual it should not be a problem from Fuso Truck side.

- This is the reason for that document and avoid a case by case decission through Fuso Trucks.

My articulated loader mechanism were "installed" by a trailer builder - he screwed the mechanism on the original flanges and painted it black after some minor modifications.

Initial registration/certfication was a 30 minutes show onsite - and some hours on the various offices.

Effectively in a day i had my licenense plate on the truck.

Some thoughts:
using ther original hooks for fixing. Screw size (diameter is limitted by the hooks - drilling not allowed) => screws = lowhanging fruit - take 10.8 quality fine pitch with locks.

Number of fixtuers = also given by design !

potential longitiudial movement => if there is the rear bracket installed make a form fitting big designed hook /no move forward / or backward.

In the front install in the first flanges rubber blocks to give way for the cabin /because twist of vehicle aigainst cabin (described in the body builders manual)

before doing all mechanics - make sure that the assesor is your friend !

Some cabins need a intermediate frame for keeping twist out. If you have this frame done and installed take this to your assesor and go over it, because he can better judge that the whole thing will be ok,
If it helps make on the intermediate frame a simpley plywood box in th future dimensions with a door  (in the back ) and test if this can be already get registered as camper. Maybee you need a window - but here you might take a cheap used one from a worn out camper => only the e Number is important, because glasses need to be compliant with traffic rules....
At least in Germany you need a bed, a cabinet a table and a place for cooking for registering it as a camper.  Stove, Bed and table can be moveable.

Some folks installed such a dummy cabin and registered before the changed to the real one. On the question why cabin has been changed - well the design was not that good then expected - we had to change it.....
Sometimes a change can be easier.....

Greets Daniel

aha, now gets more clear !

Just an idea, what would be if you register the car with a (temporarily) closed door from Box to cabin.
When it is registerd i quess nobody would care if you change the rear window.....
I would not know how they would put a "non cutted cabin" (of Fuso) to the registration papers...... That is ridicoules. Also a Cabin connection I havent seen as part of official registration. That would be way too much.......

From technical point this section you need to cut is not having any static function.

An other (crazy) idea:
make a frame wich has the shape of your needed door plus the original window and cut exact along a bead (cuenta) so that it optically not show up 😉
Inside you can have anything.... Tell that there was a defect you have repaired.....

Normaly you would frame the door anyway by a rectangle pipes for better installation of the harmonica manifold.

If you want to do it official, you probably can refer to other similar makes inside the EU without issues. (refer to the principle of equality / el principio de igualdad)
I think there are some photos here in the Forum