That's a swett lookin project.
I can't offer any tips about design, but I do have some experience with firebricks.
It seems that the splits are more easily available (about 2" thick) but that isn't the one you are looking for I think. The full bricks are the size mentioned in above posts. Call around to local brickyards where you should be able to get them for about $1.50 or so. Fireplace stores will rip you off - one place wanted to charge me $7 per split! Also check at a few brickyards if you can. The quality varies, and there are differences in temperature ratings too I think. When I bought firebricks in Iowa, they were higher quality than the ones I bought in Houston.
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