Instead of concrete to line the bottom of my forge like this (see pic) I Just used mud from the yard mixed and topped with wood stove ashes, its been in constant service for several years now. If I want to change the shape or dimensions of the fire-pot. I just move and re-pack the ashes and move the firebricks, this allows for quite a bit of flexibility, and has the added benefit of being free Oh and don't worry about trying to bolt your air grate in, gravity has been working fine in mine for years, been using heavy cast iron floor drain covers, they are about $5.00 ea at my local home improvement store, I haven't burned one up in a couple of years now but YMMV...
Jens