So first off, not bad. Not sure it would work, though.
The sub won't survive long. Chances are it'll break before you can even get one Iron Defense up. And besides, why would the opponent LET you set up the Iron Defenses? Even with one, non-STAB E-Quake will probably OHKO you because of the lowered health. Maybe you could survive. Also, why would you use Reflect? 3 Iron Defenses is enough, believe me. Try Light Screen as it will boost your team as well and will boost that "just-over-300-max" Sp. Defense. Also, make sure you lower his HP as much as possible to reduce Sub damage. Although that would ruin the point of lefties.
Overall, Magnezone has not much potential as a wall. In stats, sure. But in moves? Forget it. It's just too weak with that weakness to STAB E-Quake and EVEN if you get all that up (.00001 %), you could easily get OHKOd by a special Fire move, Earth Power, or even a powerful Water type move. So, basically, don't think about it. You could try, but for competitive batting, not really. THere are MUCH better walls out there.