That should be about right. Some guys make them too deep and then have trouble pushing the lid out of the way. What do you plan to use for a lid? I always used half inch plywood and made them about half the width of the hole with extra on the ends. Light enough so I could carefully move them around as the honkers circled. Putting enough camo on the lid but keeping it light was always a challenge. Drill a couple finger holes in the center to grip it while moving. The subfloor is a good idea. I was going to suggest dumping a bunch of crushed gravel in the bottom but subfloor will probably work better. Get some of those big holy grip treads from Home Depot, cut to fit, and screw them to the floor. That would work great for traction. You're gonna have a Cadillac by the time you're done. Will the farmer be seeding corn again?