Is it okay to build a fire pit with concrete?
You should line it with firebrick or (since you like molding and casting things) castable refractory. Edit: on second thought, stick to hard firebrick - most castable does not like to get wet. Regular concrete is prone to spalling &amp; cracking in direct fire exposure.

Depending on the shape of the "pit" a 2" layer of sand can work, if it will stay put (ie, if the "pit" is rather shallow)