You could probably find a vintage brass mat that might match the tarnish line on the plate, the style is called non-pareil. The plate is a bit undersized so might need to be secured to the back of mat with some filmoplast p90. Make sure the cover glass you use is exactly 2.75 x 3.25", if it is over the size of the format for the case, the plate package will be tight in the case, you might get it in but in time the rails of the case will break away from weakened joints.
Most people use filmoplast p90 tape, some use metal foil backed material like marbleseal but whatever you use you have to be conscious of what an oversized plate package will do to the original enclosure. Filmoplast is not a gaseous barrier so as well as finding a mat you should get also a vintage preserver.
People favor different techniques for applying the sealing tape, conservators apply it to a side of the plate package at a time with overlaps. Vintage seals tend to be one long piece with one overlap on the edge, both techniques fold over onto the back of the plate and in most cases onto the top of the cover glass by 1/8". There are vintage plate package holders that clamp the plate and glass together while allowing it to rotate the whole plate package. A plate package that doesn’t have sealing tape folding over the top of the cover glass or a preserver, presents a problem when being removed from the case. The cover glass will come off leaving the plate and mat still sitting in the case. Vintage sealing tapes were applied wet with adhesive of gum arabic amongst other things.
Before sealing clean the glass well – french chalk and a little water removes all traces of grease etc. Use a hand blower (avoid canned air as applied to close these can leave a fan shaped abrasion) to remove all specks of dust on the plate and on the underside of the cover glass. There is nothing worse than sealing up a plate only to see a unsightly speck entombed in the plate package!
