I'd speak with clearbags.com over the phone. They can do custom products, but I'm not sure if the cost is prohibitive at lower quantities. Regardless, they have premade bag model B4635 which is a 35 1/4 x 46 7/16. Unfortunately this large, the adhesive is on the flap rather than the rest of the bag. If you needed a hard backing, you'd either need to have them custom cut some and ship it to you by truck, you could buy sheets of matboard and cut them down, or you could use foamcore. Not inexpensive to do! As a side note, since you're talking about storing, you should make sure if you use a backing that it's acid free and archival. I'd be tempted to see if I could do this without backing.