Judging by the lack of replies this is not an easy item to find.
However, I made two and they fit perfectly; they are for my son's boat and he loves them. In case anyone wants to make their own drop in boxes for a 180 Sportsman, or any other model, here's how I did it:
I went to a hobby shop and bought 12" x 24" sheets of .125 styrene made by a company called Evergreen Styrene. I also bought Evergreen's .125 quarter round strips. I also bought a liquid cement made by Plastruct called PlasticWeld. Believe me, PlasticWeld is just that - after it sets up if you can separate it I will get out of your way in any back alley.
I undersized the cutouts for the bottom by 1/2 inch each way since the openings have rounded corners and I wanted to make the units square. Once I had the sides and bottoms cut, I joined the sides using the quarterrounds at the corners and where the bottom and sides met for reinforcement. Then I added 3/4 inch lips around the top, again reinforced with the quarterrounds. The oversized lips assured that the boxes would not drop into the opening. All joints were doused with PlasticWeld using a small brush such as an artist's brush and then left to sit overnight.
Total cost was about $50 per box, and the extra storage is well worth it.
I hope this info helps someone.