suggestions

I've wondered the same thing. I wonder if the cinder blocks, when heated, give off an odor or gasses that are not conducive to great barbecue... I know many use them for hog pits, but I'm not sure if the cinder blocks get that kind of hot, in that scenario.

Surely someone here will know.

I guess it wont hurt to put in fire brick but we cooked on one for years at a restaurant that did not have them. It had a large walk in metal door that did not seal very well. I am not sure if the concrete blocks were filled with cement or not.

Most are probably going to tell you to get the fire bricks and I guess if I were going to all the trouble of building one I would to.
Dave

One more layer of insulation certainly couldn't hurt.

Fire bricks except the insulating type are a heat sync. The concrete block does the same thing. Save the bucks and buy a pig.