You could try to find an eBay seller willing to ship a surplus Molle frame to you. Even domestically they cost as much or more for shipping as they do for the purchase price. Or you could try purchasing directly from the manufacturer, but that's not exactly cheap.
https://downeastinnovation.com/product/dei-1603-molle-large-frame/Either buy the shoulder straps and belt at the same time, or jury rig whatever you have, it's pretty versatile. Then it's just a matter of sewing a piece of canvas between two web straps, and attaching that assembly to the frame with four side release buckles (2 top, 2 bottom). Or you could use tri glides on the bottom.
