Hi Kate,
You can cut a 2-ply archival window mat that has a window that is slightly larger than your inscribed paper mat. Ensure the outer dimensions are the same. Place the archival mat between the photograph and inscribed mat. Use an archival 4-ply board for the backing in the frame. Use archival paper (Light impressions) to make photo corners or buy archival corners to prevent applying adhesive to the photograph and attach to the archival backboard using gummed linen or archival paper tape. Conservation frame shops can easily supply the archival matboard and that way you know your barrier layer will be of good quality.
Amanda