Looks pretty good to me!
Do the sizes of the heating mats match the size of your growboxes?
Get yourself a couple of Sterilite boxes at Walmart: if you buy one that's 12-13 inched high as a growbox (they hold exactly 4x5=20 4x9 pots and about the same number of cups) and one that's about 7 inches high as an upside-down cover, you're good to go!
IF I use cups, I always use the lids too: cut them once to the center, and you can easily put the lid around the cutting (or take it off). Use Painters tape to tape the cutting line and secure the lid to the cup.
Happy rooting!