A few years ago I built a Ventura
door from scratch, only reusing the aluminum external flanges/framework.
It looks like they started with a typical residential hollow-core door slab and just rough-cut for the lock and window.
The difference was the thickness, rather than 1-3/8" it was 1-1/8" - that's my memory, don't take that as fact without measuring!
Anyway, I sawed wood strips to the appropriate thickness and glued and nailed the pieces to pre-cut wood panels, paying attention to reinforce around the lock and new (fixed/non-opening) window.
For the exterior I sourced a sheet of powder coated sheet metal.
Overall length I shortened by 1/8" to allow an air space and drainage gap at the bottom flange to reduce moisture buildup and rot. I cut plastic spacers to make up the difference.