I didn’t use Outer Shell to give the cones thickness. I drew them that way. I started with a profile shape for the cone and added some for the base plate. This was drawn so the apex of the cone was at the center of a circle.
There are other ways to draw the cone, too.
Select the circle, get the Follow Me tool, click on the profile and the cone shape is made.
I drew a square centered on the bottom. Hit Ctrl after getting the Rectangle tool to draw the square from the center.
Push/Pull the face outside the square to get rid of the excess. Make sure you don’t have any holes and make the thing a component. It should be solid.
I repeated the process for the other two cones and made each of them a solid component.
As I described above, I made an array of just one size of cone. This was done with Move/Copy. Then I replaced some of the large cones with the middle sized ones and some others with the small cones. Note, the cone cells must be components for this to work. It does not work with groups.
After all of the cones were correctly sized, Outer Shell was used to combine them into a single entity.
You might find for small stuff destined for 3D printing it is better to work with units set to meters and just import the STL into the slicer with units set to millimeters.
Many slicers could handle printing the whole thing without actually combining them into a single solid entity. Here I’ve added the square holes at the corners and uploaded the STL to i.Materialise as a test.
The cool thing about leaving the cells as separate solid components is that you could quickly and easily replace some components with other ones to change the project.