Three solutions for body mounting:
Glue the box to the floor. Testor's cement didn't work when I tried it, but there should be something that will.
Pin the box to the floor. Cut the pin from the lid, drill a hole where it was, and fasten the box to the floor with a pin through the hollow center post. You MIGHT be able to drill it out for a screw, but I didn't try.
Make a new box. Styrene strip and rod would work, and you wouldn't need the entire box. A post, a cap to keep the coupler on the post, and two side pieces for the springs to work against. You might even find a screw the right diameter and not need the post at all. I installed Accumates on flatcars that way, using a piece of aluminum tubing for the post, drilled out for a 00-90 screw, and a brass washer to keep the coupler flat. The end sill worked fine for spring posts.
I never did use the IM boxes, as the idea was to avoid making body mount boxes for the Accumates, and they wouldn't work in the IM boxes... I'd replaced the truck-mounted Accumates with body mounted MT 1025s, and wanted to reuse the Accumates for extended draft gear and on cars that sat too low for 1025s.
