Would there be any reason not to use plastics like Acryl, Polycarbonate, Polysulfon or other transparent material instead of glass ? It would save some weight and withstand shocks from accidents.

The only reason that comes to my mind is that, unless very thick, those materials may not be rigid enough to keep the paper and negative together.

Plastics attract dust like nothing else and it scratches easily. I would avoid it and not worry about breaking glass. I've never broken glass in my frames and I'm pretty tough on equipment. Even if you do break it, glass is so cheap it's nothing to replace. Go to Lowe's and they cut it for you for less than $2 for an 8x10.

what Bruce said plexi is a dust magnet and scratches if you so much as breathe on it. I only use 1/4" glass in my contact frames, although single strength is probably fine.

I imagine not much different in Denmark, but glass shops here can provide finshing on the edges to minimize chipping.

A simple smoothing (flat) or a slightly more expensive 'pencil polish' which gives a round edge like one might see on a table top. For a pattern cutting charge, you could have round corners.

The weight of glass is really beneficial to keeping flat contact, for contact printing, if you don't have one of those contact printers that relies on a locking clip.