I've been buying quite a bit of material from Bruce Baryla's Civil War Sun Tax exhibit over the last several months, so I've been looking for a good way to store the CDVs (carte de visites). I want to be able to protect them, since they can be somewhat fragile, but at the same time I want them to be accessible and visible. If someone wants to do a closeup examination of one, I want them to be able to do so without worrying.
Normal mylar cover holders that I use for checks, receipts, and other documents won't do the trick, as they are not protective enough and it's far too easy for the CDV to fall out. Also, some CDVs tend to be fairly thick and/or heavy compared to documents.
I thought back to when I collected sports cards back in the 1990s, and figured that archival toploaders (rigid plastic cases) would do the trick. There's only a few problems with this approach:
1. Your standard 3x4 baseball card toploader is a bit too short for a CDV. Carte de visites have a different proportion from sports cards... they're taller.
2. There's the potential problem of abrasion against the toploader when the fit is snug.
The solution is 3x5 toploaders with polypropylene sleeves.
The sleeve is slick and allows the CDV to slide in and out of the toploader without damage, and since it is acid-free, it is good for long-term storage. The 3x5 size is large enough to fully enclose and protect the CDV, while still allowing enough space for me to place my collection inventory sticker on the picture side (the back side with the stamp and backmarks is of more interest to me).
It's solid and secure.
The following is an example.

