I have an object which is to be 3D printed. This object will then be assembled with other objects. However, due to the 3D printer being not very exact, I need to add a small tolerance to the object (i.e. make the object a bit thinner) so it will fit with the other objects. I would also like the base of the object to be slightly thinner than the middle and top, to offset the object being fatter at the base where the first few layers are printed.
Here is a screenshot of the object:
Specifically, I would like to move all horizontal edges at the top of the object slightly "inward" on the XY-plane by a constant amount, and move all horizontal edges at the bottom in a similar way by another (larger) constant amount, while preserving the height of the object on the Z-axis. All edges that are currently parallel to each other should remain so.
The effect would be that all points inside the modified object are also inside the original object, and the hole in the middle becomes slightly larger.
Note: The Alt-S command does something similar to what I want, but it doesn't seem to allow me to lock the Z-coordinates. I do not want the height to decrease.
EDIT: Here is my .blend file.