The box on bertas boy was made using the existing splashwell of the boat and fiber glass. the old owner said he just added a rear wall to the splash well using ply wood and covered it and the surface of the splash well with a mold release. Then he just laid fiberglass in the "mold" and let it cure. After that you pop it out of the splash well and trim it up as you like before adding a lid. Then re install it in the splash well.
On my old 22 we had a glass guy just glass in a rear wall in the splash well and make it part of the hull. the we added a top to it and created a nice fish box. It was one of the best things I did to the 22. created extra storage for dry or wet items. extra place to sit too. we loved it. It was not removable like the one on bertas boy butwe lover it. Oh it had a drain in the center.
