Hope the storing is going well. Mine are still currently boxed in the loft in storage tubs with lids on. Not got the space to get them out just yet.
I have protective sleeves for my SNES boxes and just did a really quick look on eBay and you can get them for big box PC/Amiga too.
https://www.ebay.co.uk/itm/2-x-PC-Am...-/222742102707
These look a little pricey and you might be able to find them cheaper, or if you buy in bulk you might be able to get them somewhat cheaper too.
Have a love for box art too though. I'm currently working on a project called WeHeartBoxArt where i want to create a website dedicated to the love of box art and it's artists. Then there's the coffee table book to go with it! Haha. Just trying to build a community on social media at the moment though.