The L shaped nestbox separates the entrance to the nestbox from the sleeping/egg chamber. The purpose is to prevent a bird from dropping directly on to the eggs or other sleeping bird. An internal shelf just inside the entrance allows an incoming or outgoing bird somewhere to rest while the outside or just before moving to the egg chamber.
It also has the advantage that when in the egg chamber the entrace is not visible so the birds tend to feel more secure because a predator cannot see them if they cannot see a predator. (thats the theory anyway!)

This design of nestbox looks like a letter L. Basic dimensions are 450mm tall (the height of the L), 150mm width at top of L, 150mm high at base of the L, 350mm wide at bottom of L, and depth of 200mm.
Constructed from 20mm thick plywood and glued/screwed together.
Opening is at the inside of the upper L, centered in the depth and about 40mm from the top, hole diameter to suit Lory but sized from 40mm to 60mm diameter. Just below the hole is a short protruding perch or a short section of 20mm square wire.
Viewed from the side of the L there is a downward hinged flap at the leftmost side of the lower L shape. This allows access to the nesting chamber. Also the top part of the lower L is removeable for cleaning out the inside.

                 +------------------+
                /                  /|
   depth 200mm /                  / |
              /                  /  | 200mm
             /                  /   |
            +------------------+    |
            |    150mm         |  O |  <---entrance hole 50mm diameter
            |                  |    |                  
            |          xxxxxxxx|    +------------------------------+
            |         xx       |   /       250mm                  /|
            |        xx        |  /                              / |
            |       xxxxxxxxxxx| /  This section is       200mm /  |
            |      internal    |/     removeable               /   | 150mm
  height 350mm      shelf      +------------------------------+    |
            |                     +------------------------+  |    |
            |                     |  hinged down flap      |  |    +
            |                     |                        |  |   /
            |                     |                        |  |  /
            |                     +------------------------+  | / 
            |                          egg chamber            |/
            +-------------------------------------------------+
                         width 400mm