Great stuff Ulrich!
I really like the tree roots idea - they look right! There are supposed to be ways to prevent wood from rotting without discoloring the wood, etc., but I've never had any of them to work very well. Maybe someone will share their approach.
Clear exposed film acetate works well for spectacle (glasses) lenses and gas masks. Round lenses are especially easy - you just need a punch and die set or a leather punch tool. I use white glue to fix them wherever. Dries clear and I've never had it distort the acetate or smear.
All the best,
Dan