E-track boxes on the market were to big and deep for what I needed.  So I made 4 versions. 4 3/4 inches with and w/o slots and 9 3/4 inches with and w/o slots.  I decided on 9 3/4 so 6 would fit on a 5 ft run (3 on a cut in half run).  The 4 3/4 if other length cuts were made to not waste the E-track slots.   Box lengths were made at the 3/4 inch marks so there would be a 1/8 inch gap between boxes when hung in the track.  Photo shown is the slotted 9 ¾ inch.