I don't think the gen 2/3 pmags are dimensionally different. I believe the base plate of the gen 3 is slimmer. Unfortunately, finding the golden ratio for your loader dimensions might require you to purchase or borrow whatever you're trying to fit, however, the AR is a modular gun so the overall dimensions of the mags will be similar. I've seen videos of said loaders and there are some tolerances built into the mag portion of the routed wood.