How I Designed This

Procedure

The general shape of the "block" is defined as the intersection of 3 identical orthogonally aligned regular octagonal prisms. To model this yourself, sketch a regular octagon centered at the origin on the top, right, and front planes. The distance between two opposite faces on the octagon is 3.23 inches. Then you must extrude each octagon symmetrically, again making the width 3.23 inches. Now, using the "Boolean" or "Union" feature in your CAD software, combine the 3 parts, leaving only the region where all 3 intersect. To make the rounded edges, use the fillet tool on each edge with a radius of 0.23622 inches.