We searched long and hard to find boxes that used the under bed storage area most efficiently and we found ones that waste almost no space at all.
Iris Weather Tight Storage Box 46.6 Quart
Two of these slide in with no wasted space at all.
Amazon.com - Iris Water Tight Storage Box 46.6 Quart - Airtight Storage Containers
P.S. we use the end of the awning handle to drag out the aft most one.
That awning handle (provided the "handle" end is gently closed a little by a bit of a squeeze) can be slipped into the hollow space between the two bulkheads at the opening of the street-side / left / port hanging locker.