I put them each in protective bags, but had been thinking for a while about getting a slipcase for them. Found a place called Everything Called Books (https://everythingaboutbooks.com/) and, I gotta say, I'm very pleased with their results.

Had a lengthy back-and-forth email conversation with Patrick there and he always responded quickly. I found an AI-generated image and made it a silhouette, provided it to him, and he created the one-off slipcase very quickly. It was packaged so well I thought I should have done an "unboxing video" for Youtube; probably the best packing I have ever gotten from ordering online.
If you're looking for a one-off special slipcase at a good price, give Patrick a buzz at that site.