All my square filters are cut from coloured Perspex, alias Acrylic, alias Plexiglas. This is the stuff that comes with the protective brown paper that you just peel off before use.

My 84mm "Cokin" squares are cut out with a hacksaw and work just fine. The surfaces are optically flat and I've never seen image degradation either in camera work or astronomical telescope applications.

The plastic surfaces are much softer than glass but a scratched filter is cheap to replace; about $1 each if I source Perspex off-cuts from the local plastic fabricator.