I tried 1/8" and it warped when exposed to heat. Stick with 1/4"
A warped deflector can still be functional. You can't sit the stone on it, because the stone won't stay level, but if you suspend the deflector below the stone, the deflector can warp a bit and still do it's job.
I know that some people still believe that deflectors can be smaller than the stone, but I strongly believe the deflector has to be a tiny bit larger to be effective. 1/4" steel plate, cut larger than the stone and to somewhat exacting dimensions to allow air flow, can get costly. If someone has the money, sure, 1/4" will work just fine, but far thinner metal can also serve the same purpose.
I spent some time researching less expensive options, and, while nothing is a complete slam dunk, for those counting pennies, I think some of these might be a bit more viable.http://www.agrisupply.com/disc-blade/p/69477/http://www.acehardware.com/product/index.jsp?productId=3512372http://www.homedepot.com/p/MD-Building-Products-1-ft-x-2-ft-Steel-22-Gauge-Weldable-Sheet-56066/202091749?N=c27v#.UmDCiGqZH0lhttp://www.amazon.com/dp/B001BSOX58/?tag=pizzamaking-20http://www.webstaurantstore.com/town-34720-20-hand-hammered-cantonese-wok/88534720.htmlhttp://www.wasserstrom.com/restaurant-supplies-equipment/Product_421222http://www.lionsdeal.com/tab-trs-6506.html
Out of the whole list, the tillage blade looks the most promising. Shipping is a bit of a wild card, but, even if it's $15 for ground shipping, that's still pretty competitive. Filling the axle hole shouldn't be too much of a problem. At 3.5 mm, it's a little thicker than 1/8" and the concavity should go a long way to prevent warping, although, like I said, warping is not really an issue.
The trash can lid is the cheapest, but then you've got to get into zinc removal, which can get pretty nasty, depending on what acid you use to remove the zinc and the direction of the wind.
I didn't come across it in my online travels, but nonstick pans can frequently be teflon on steel. If you happen across an 18" to 20" non stick pan that's thin but not that light, removing teflon, is, as far as I know, easier than removing zinc.
This doesn't spell out every possible option for deflection, but, for those taking the LBE route, I think it helps to provide some thriftier options than 1/4 (or thicker) plate.
Edit: I was at Restaurant Depot yesterday and noticed they have a pretty wide arrangement of sub $10 stainless bowls. I didn't take any measurements, but I'm sure that one was in the 20" range.