The Shellback Tactical Flap Sac 2.0 Pouch was not intended to be used with armor but instead designed as a waist pack add-on to your armor carrier. However, if you wanted to use soft armor, you must use something that fits within the 7"T x 9"W x 2.5"D interior dimensions.
Per the product info: "The rear pocket is designed to accept soft armor inserts to provide ballistic abdomen protection." Having said that, this is the manufacturer's soft armor insert: https://www.opticsplanet.com/shellback-tactical-level-iiia-flap-sac-soft-armor-insert.html