Buying a custom GX mat and cutting it is not practical. The high cost and the required modification with cutting the custom fit would compromise the outer seam or molding look.
I think a general mat would fit your need better.
I have a mat in my GX that is made for a garage.
I think this is it.
I expected to cut it, but turns out it was not necessary for my application (it covers the entire rear).
This type of mat allows a cleaner look when cutting is needed.