I have a manufactured "stone" patio with poly-sand to fill the grout lines between the blocks. I want to add a fire pit to this patio. I envision a ring of blocks several courses tall to contain the wood. What kind of bottom do I need for this fire pit? I would expect the fire to discolor the patio. My main concern is how to deal with the ashes in the bottom of the pit. I don't want rain water to wash the ash out and across the patio, leaving a mess for me to clean up.