Yup.... I'm using Bainbridge Alpha Rag 2 ply mounting board that you sell. I stock few sheet of these here.

Unfortunately, after a day in the frame, 1/8" foamcore board (purchased elsewhere locally) that I decided to use seem to have buckled that window opening is no longer sitting flush with the image. I now have a good idea on what may have happened. I'll be fixing it tonight.