Very important: the work should neither be put in contact with the glass (acid), nor with an acid wooden or cardboard frame bottom.
My advice is to use:
1/ a non-acid ivoiy laid cardboard passe-partout in 40 cm x 50cm format, with a 22 cm x 29 cm or 22 cm x 32 cm window, between the work and the frame bottom,
2/ a non-acid bottom cardboard between the work and the frame bottom,
3/ a frame moulding of your choice ( i.e. a plain white or natural wood frame moulding with a simple profile, and not too wide ), for a 40 cm x 50 cm frame.