Years ago I made a camera obscurer for my wife's art class at the local elementary school. It was the same idea: lens created an image that reflects via a front surfaced mirror to a horizontal ground glass. Finding a lens long enough was solved by use of a Jeagers telescope objective. the result was a big box. Apparently the teacher and kids were thrilled by the resulting image on the ground glass. They could trace, just like the old masters did, to ensure correct perspective for a painting.