WWII Luftwaffe Secret Base (late war/what-if?)

The idea behind this diorama was to build a hidden/subterranean base as a store for some of the experimental and prototype fighters of the late war - some of the “what if?” designs which never made it to production.

After having gained the experience of working on the WW2 French countryside ambush and WW2 German Airfield display, the idea of a larger, multi-level diorama hosting several of the advanced jets and rocket the German airforce was working on really appealed to me.

Two level - upper 'ground' level and a lower maintenance/storage area.
The upper level will be both 'open', with a runway/taxi-way section, and the 'hidden' entrance to the base in a hill or cliff face. Space for at least one model on the taxi way, and one more in or entering/exiting the base entrance.
The upper level should be removable so that the upper and lower levels can be assembled seperately
The upper level should have a removeable cover over the entrance, so that models can be placed and then the hill/cliff/earth cover added
The lower level will have space for 3 or so fighters
The lower level will have a suspended walkway leading to a control booth overlooking the maintenance bay
The lower level will have a narrow gauge rail link, to transport in parts/fuel/ammo
The lower level will have a crane to offload cargo from the rail cars
Some working industrial lights (probably LEDs) hanging from the ceiling of the lower level and as wall lights in the entrance of the upper level - to work off the existing LED strip illuminating the cabinet

A very early sketch of the multi-level layout:

Design - Upper Tiles

The upper tiles have the following features:

Left tile is a runway, edged by grass and a small number of trees - it needs to be wide enough to place the widest model which I will use; Revel 1:72 Arado AR 555 (wingspan 29cm, length 22cm).
Right tile has a short length of runway connecting to the main runway and the entrance to the base. The width of the widest fighter which will enter/exit the base is the 1:72 Revell Ho 229 (wingspan 23cm, length 10cm). This means that the entrance 'door' needs to be almost full width - therefore there won't be a support on the front (nearest) edge.
Right tile is topped with a (removable) grass/earth roof to show that the entrance is recessed into a hill or cliff.
Entrance door will be made in two versions - completely closed and partially opened; it will be designed to slide in from the side or to be removed completely
Small section of raised walkway inside the entrance
Several LED lights inside the entrance
Another (non-moving) door recessed into the far right wall of the entrance, to indicate a lift down to the lower level

More detailed plans of the upper levels:

Fig. 1: Upper level, left tile, plan view. Fig. 2: Upper level, right tile, plan view.

Fig. 3: Upper level, right tile, side view.

Design - Lower Tiles

The lower tiles have the following features:

'Concrete' floor throughout
Left tile features section of rail (using OO gauge) for delivery of parts and spares - this is lower than the floor level
Right tile has a recessed door, to the same dimensions of the door above it on the upper tile - 'off screen' there is a motorised gantry raising aircraft to ground level.
Lighting suspended from i-beams/girders that 'support' the roof (in reality all weight is transferred down through the 6.5mm Plywood sides and back panel).
Suspended walkways on the left and rear walls
Personnel doorways on the rear wall in line with one on the upper tile - 'off screen' there are stairs behind the rear wall to go between floors.

This excellent diorama from uschivdr.com also gives an indication of what the lower level is based on:

Fig. 4: Example maintenance bay from uschivdr.com.