On early versions (1973-1978) of the famous Ferrari 3.0L V8 engine, there is nothing allowing to lock the camshafts (without having to remove the head covers) when having to remove the pulleys that are tightened with the important torque of 11mkg. It corresponds roughly to a pressure of 150kgs applied on the 8 teeth of the tool.

So I decided to study a tool for that. It supports an angular position of the camshafts of +/- 6degrees

Here it is of the machine

1/ The two parts slide on the pulleys axially (in the direction of the camshafts).
It is necessary to put the teeth in front to manage to put them in place because it is well adjusted.
After that they are 'hooked' to the pulley which makes positioning the other two pieces very easy.

2/ With the belt in place, there is only one valid position for tightening and another for loosening. You have to think about the direction of thrust when setting it up.

3/ With the belt in place, it is thanks to it that the two pulleys remain in relative position during tightening/loosening.

Without the belt, the tightening of the two parts modifies the angular position of the camshafts until the two circular moving parts come into abutment on the fixed parts (see photo)

In this case, after tightening the screws to torque, the camshafts must be put back in the correct position using the marks located on the camshaft supports for the pulley, see photo.
They must be on the point of tangency, and therefore spaced 141mm apart.

4/ In both cases, the torque of 11mkg can be applied. To test, I started at 5mkg and gradually increased to 11mkg. I don't think the radial forces applied to the camshafts can damage them.

Conclusion: this tool allows you to lock the camshafts for the sole purpose of loosening the screws to remove the pulleys WITHOUT having to remove the cylinder head covers (and all what that implies)  

Another even simpler and perfectly secure solution would be to make an almost complete crown leaving a 29mm opening to slide it around the camshaft.
One of the advantages of this tool is that its installation can be done every 12 degrees (angle between each tooth) and therefore allows easy installation whatever the pulley and there will always be a solution to pass a long handle.
I do not think that a nylon realization by 3D printing is sufficient to pass the couple in the fixing holes of a handle