AND its looks good to, to use stone.... 5x5 then a "ring" on the outer end, then fill the 3x3 inside whit like dirt... and a 1x1 in the middle.
Then cover the dirt whit stone, leave 1x1 so you can take out the dirt ( so you can use that space for the wood ) and then you just block it whit a 1x1 stone and let it "cook"