100 Pack 13" x 18" Plastic Resealable Grip Seal Bags - Airtight Food Storage, Organizer for Snacks, Nuts, Candy, Jewelry & Crafts - Durable Zipper Lock Bags for Kitchen, Travel & Office Use

I use these bags to give moisture protection to my collections of old magazines. The bags are big enough to cope with A4 size and also a size wider but shorter than A4. You can get plastic boxes of the right size (not too deep), from Lakeland if my memory serves me well, but they are far from airtight. I use the bags inside the boxes. I have not subjected these bags to scientific tests, but the seal is very good, if a little tricky to use. When the bag is properly sealed but with a little excess air, you can press the sides and the top comes up, press the top and the sides bulge out. Good enough for me.