References will be from standing behind trailer looking forward.
The vertical piece that runs from left to right (in front of the blue foam) is screwed (from the front side) into a piece of wood which sits under the foam. These screws are more than enough to support that vertical piece from forward rotation. That piece of wood is cut, on its bottom, to match the taper of the rear of the compartment, and screwed straight down into the taper. Yes, that vertical piece of plexiglas runs all the way to the left side, but no additional support was need at the left end. Nor is the right end support with any thing on that wall. What you can not see is the the bottom piece of plexiglas under the blue foam. That piece sits horizontal on top of the tapered piece of wood, and is dadoed into the vertical piece and runs all the way from left to right also. I can get my hand all the way into that back left corner, and fit three cans in that space. You can see the small support i placed under the shelf on the left side of trailer, behind your red arrow. The shelf, with its back and sides is one unit, dado and glued together. Front left corner had to be cut at angle to get it in, and now it fits snug. If it starts to move around i planned to screw it down into the piece of maple on the front of compartment supporting the shelf. That piece of maple has the slot cut in it to accept the vertical retaining piece. The vertical piece of plexiglas has a slot cut in it to match. The vertical retaining piece comes out for access.
Hope that explains it.
