If your cast off bag has a front flap, you could add a second soft panel and expand the coverage to 15 x 22! A lot of back packs now have a space for a laptop that easily accept Kevlar panels too. I gave several out of date armor panels to friends to cut the ballistic panels to fit their kids packs. At this point they are not certified but it was a good use for them. DRI have a cast-off computer bag for a laptop with a 17" + screen with an 11X15 Level IIIA Soft Plate in it. You can use the shoulder strap to hang it around your neck if need be.
.SOFT body armor inserts are usually called "Panels".
You can buy the Kevlar material on E bay, and make what ever size or shape you want. The only problem then is you get no testing or certification with it. Its not hard to cut. I think the standard for 3A protection is about 11 layers. but that would depend on the weave of your material. DRI guess I'm going to have to research the subject of "cutting" soft armor. I wonder what that does to change the properties of the panel? I guess I asked myself a question that will result in some homework.
Age will degrade its rating, I have shot at several vests and panels that are out of date, and not one has failed. What does make them fail is over cleaning. That's why the panels are removable from most vests so you can wash the carrier. Constant sweating in them means you have to get them cleaned or they would develop a smell. But hanging in a closet should not hurt them. A lot of departments donate their old vests to cops in third world country's. The vests still work for them. Yours hanging in a closet should be fine. DRMy cousin was with a local city PD back in the 80’s and then transferred to the County SD. The city didn’t ask for his vest back so, he gave it to me. It’s a Safariland Level IIIa, I believe. Stops up to .44 Magnum rounds. It has pockets front and back that accept steel/ceramic plates if you want to upgrade. Question: It’s been hanging in the closet, will age have degraded its protection?