Seems to me you'd want to hold onto as much structural support as possible, especially if you are hanging a heavy engine on a bracket. The beds are filled with foam, which absorbs water and does nothing structurally. Here are my "knees", tied into the engine beds. One disadvantage is that water may accumulate outboard of the beds as there is no drain to the bilge. My knees are NOT a good example of proper design and glasswork.
In the following image, the humps have been removed from the engine beds, creating a nice flat surface for a removable floor that I made from a sheet of synthetic board.
The unfinished hatches...
The finished hatches and storage:
Some images of how others did it from my files. This first one allows drainage to the bilge, which is good.
