Ditto on the advice so far. On my Explorer UL and my son's Expedition I had the 2QZQ mod 3. This gives me the best of both worlds. Hennessy uses kind of an oversized and clunky zipper. 2QZQ uses a much thinner/lighter/better model.
You have the option of having the bottom sewn shut when you have the mod done. I figure having the bottom entry is a good think in the unlikely event of a zipper failure. It's also the faster way in/out in really buggy weather.
While you're at it.....they also make some really nice organizers and other accessories