On one of the legs go into poly subobject mode and select all four groups of polygons with a height of 15 (created by the extrusions above) - in total will be 4 groups x 4 sides = 16 polygons. Extrude the selection 3 units By Local Normal.
Now we'll need to select some edges to do a chamfering. First, in the Top View, select all non-diagonal edges (2 marquee selections - top-bottom and left-right - will do that).
Hit Ctrl+I to inverse the selection. This is the selection we wanted for the next step.
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