I would create my main body with a “Center Rectangle” on the Right Plane with the center coincident with the origin.
See Step 1
“Extrude” using the “Mid-Plane” option, this keeps the part centered on the origin.
See Step 2
Start another sketch on the Right Plane and this time have the center be coincident with the Front Plane and dimension it accordingly.
See Step 3
Now do a “Cut Extrude” through all in both directions and voila!
See Step 4
BTW… This may have a better home in the “Parts and Features” section of the forum.