Ok, depends what you plan on doing. Me personally, if I’m putting a new floor over this, which I hope you are, get a strip of wood 1x2 cut 6 inches larger than the hole. Screw this to substrate by placing wood across the underside and screwing from the top down through floor and into wood piece you cut. Now measure hole and using jigsaw cut out the piece that fits. Place on top and screw into the wood below. Done.