Using multiple infill types within one model When modelling for 3D printing, can I distinguish somehow the type of infill in various areas of the model? Say there may be some areas in the model where I want 100% infill (maximum strength) but some areas where the infill can be less (maybe 25%).

I am new to 3D printing, doing my first model (enclosure for electronics - camera module). Using tinkercad.com only so far. Places where 100% infill is wanted are usually walls of the case and "threads" for the bolts but I want to make the enclosure as stiff as possible as a whole so I am thinking about using some sparse infill in the "free space" inside of the enclosure (so that it does not break so easily when it falls to the ground etc). I just do not know how to define this within the model.
To achieve additional localised stiffness, you can also insert small voids (gaps) inside the model. These become double thickness walls once sliced and can be used to support things like screw holes.

See the 'negative' parts used with a cube, and the sliced result here: