NLK,
All mounting screws attach to adjacent cabinetry. None into the B+ dome because like you say, the screws would go through to the outside. Also, the location for screws are in places where they don't pierce through into an adjacent cabinet interior.
It's been over 3 years, but I seem to recall pre-assembling my custom book slot cabinetry, then work it into the PC cabinet. It is screwed only into the floor with 2 screws, and side wall of the cabinet with 1 screw using flat head counter-sunk screws, the 3 favor the front for ease. Screws were located where the opposite side would not show. You don't want to see a screw coming out into view or into another cabinet.
The top shelf is a glove fit to the B+ fiberglass dome. That shape is odd so I had cut a pattern of it from cardboard then transferred the pattern to the board.
The vertical slots have a little gap to the back wall because of the curvature there.
The assembly is well fitted to the wood side wall of the cabinet.
There are a couple inches between the assembly and the front. This makes getting things in and out from it easiser, and also makes it possible to get the pre-assembly into the cabinet in the first place.
------------------------------
About the right side shelf with power strip.
The shelf is shaped like the other side, done in the same manner with a cardboard pattern.
There are 3 stubby "L" brackets that screw into interior wood in a tri-pod placement. The power strip is mounted to the shelf itself, and is plugged into the PC supplied outlet there.