Hi Gabby, we used to put glass or acrylic behind fretwork where I used to work and it is fine, only thing I would say is it can be a pain in the butt to cut. Also you will have to use an adhesive which will not remove the silvering from the back of the mirror if you intend laying the mirror into the frame. If you're fixing the wood to the face of the mirror, heavy duty double sided tape will do the job. The adhesive we used for the silvered side of the mirror was Evostik Mirror adhesive. I am quite sure you will have something similar on your side of the POND. Good luck from Scotland. RR