3D Mini: Hole

Make a cube and change . . .

Scale Y:  0.25
Subdivisions Width: 3
Subdivisions Depth: 3

Select the center faces and delete them.

Using the Append Polygon Tool, attach the bottom edge to the top edge of the hole on all four sides.

Mesh Tools > Append to Polygon   Select the first edge, then select the second edge, and then hit enter. You can try the Bridge Tool with Divisions set 0, but it may twist funny because it does not know what edges to connect.

Select all the vertical and horizontal  edges and bevel them with an offset of 0.015 units. *Make sure absolute is still set as default or set Offset As Fraction to off.

NOTE: If you were to bevel just the inside edges, the topology would not flow when smoothed due to the triangles in the corners.
 Cube-2 Cube-3
NOTE: If you were to bevel just the inside edges with 2 segments, the topology would still not flow when smoothed even though there are no more triangles in the corners.
NOTE: See how the geometry looks when smoothed
For clean topology, select all the inside edges and the connected edges.
NOTE: This tie notice the N-gon or a 5 sided corner, we will fix this by adding a 2 segments. 
Bevel the selected edges with a 0.015 offset and two segments.  It should look like the image below.
Select all the outside edges and bevel them with a 0.015 offset and TWO segment.
It should look like this.
And smoothed like this. *Don’t forget to apply a Blin material. 
*Change to Blin.   Right Click on the object and Assign New Material.  Choose Blin.

TURN IN 2 shots, both smoothed one selected showing wire frame.

Repeat last few steps and extrude the center face 0.25 units in the Translate Local Z.