As it happens, one of my hobbies is model building and over the past two years I've become well acquainted with 3D printing. Here's my 1/87th scale model of the first steam locomotive built in the U.S.
Best Friend by
Berang Berang, on Flickr
It also just dawned on me that the Holga uses a 60mm lens, but I might seek something a bit more "quality".
I suppose the shutter blade could be made from laser cut brass, and if I keep with the box camera theme, it could be done with a single "blade" and a lever for releasing it, one exposure in each direction the lever is pushed. I could probably even just steal the dimensions/geometry from a box in my collection.
The plan for the film advance is to use a double red window. I got the idea from my Koroll 24S, which makes 24 exposures on 120, but which regrettably has a film gate giving an aspect ratio identical to that of 135, instead of using the full width of the 120 film.