Preserving PL Sets From The 50s

The plastic freezer bags or similar bags are likely not a good idea. Mylar is safe. Do you have a picture you can post? I would shop at a hobby or coin shop for a similar sized coin holder in a clear sleeve and only use the sleeve.

You might consider purchasing the inert sleeves that are used to store paper money. They come in a variety of sizes and styles and offer reasonable protection while still allowing transparent viewing of the obverse and reverse.

I use them to store my collection of cardboard holders that were used to house and deliver the US commemorative half dollars back in the 1930s. Most times the holders are empty, but some of them contain coins. The sleeves have protected these pieces in my collection very well.

Stamp dealers sell similar sleeves and are an option as well.