UDPATE I upgraded this model to a working 3-d-puzzle.
It came to my mind, that I had created quite some simplified plane models by now, but not one biplane. The construction needed to be a little more complex, but it's definitely worth it.
My build was done on a desktop cnc-machine by Stepcraft from 6.5 mm beech plywood using a spiral-toothed 1.2 mm flute. Glued together and nicely sanded it's a great toy for boys (of different age).
A laser cutter might be a good alternative tool to create smaller versions.
Visit https://www.zenziwerken.de/en/Toys/Plane-Models for more simplified plane designs.
Keep in mind, that the stl-file is just for validating the construction and viewing in the browser.
I searched 3-view drawings of this plane to create the outlines. I traced the images in Illustrator and carved out the main features. The slots were a result of trail and error. Before building the plane I did a 3D validation in SketchUp. Then I nested the parts as close as possible for production on a cnc-router.