What does a fuselage frame structure of a non-pressurized transport aircraft look like?
A pressurized aircraft structure differs from non-pressurised in that the fuselage center area, including the cabin and baggage compartments, are built as a pressure vessel.

Except for:


strengthened frame and windows
forward and aft pressure bulkheads
pressurization systems


the aircraft structure and design not related to the pressure vessel (like the floor) remain unchanged.

There are many examples of non-transport civil aircraft that have pressurised and non-pressurized versions.  For example, a Beechcraft "P-Baron" or a Cessna P210 Pressurized Centurion has smaller windows and different cabin doors you can easily observe.  Yet, the design principles of the aircraft structure, besides beefing up the pressurized areas remain the same.

Most non-pressurized transport aircraft are utility or military in nature, as is the DeHavilland aircraft you mentioned.  Design considerations for these aircraft, like the floor you mentioned, are related to their mission, not pressurization.  That is, unless the floor itself is a pressure vessel wall.