In order to work on reeds, you need a stable surface, and an acrylic plaque is perfect.
You can sometimes pick up scrap acrylic that you can use for working on reeds at craft shops. That is where I got my first one – there was a craft shop on the walk between the Eastman University Avenue dorms and the school downtown, and I still use it. They last forever and they keep your knife from being damaged because the material is softer than glass, for example, so don’t use a glass plaque. You can also sand on it.
While you will notice that the acrylic surface gets marred from sanding and using a knife, you will likely never see any acrylic debris – I never have – plus it is plastic and inert. Also, if you hold your reed in the same place on the plaque, it will create an outline over time which tells you where to place the reed.