Contour (maybe macro) question

Good Day All.

I need to regularly draw an inside contour in a rectangle that has different spacing on at least 3 of the 4 sides. Also needs to be on a separate layer.

For instance, in a 48 inch wide by 36 inch high box I need an inner contour with 8.5" bottom, 12" top, 4" left and 6" right spacing. I can do the math and work it all out but chances are that when doing 50 different ones, I will make at least one mistake. Likely 2 or 3. Since these are for parts that will be CNC cut and sent out, an error is expensive.

What are my best options for this?  If it's a macro, where do I find a place that can write one for me?

Thanks in advance.