Search around for tuning plates and convection plates, I did a baffle and convection plate, its in the wood smokers forum, still on the first page. I posted a picture of the horizon baffle plate which may or may not work for you, they do 16', 20" and 24"
The way I understand it, its a dampner if it starts in the firebox and goes through into the smoking chamber, if it is in the chamber only its a baffle, either way its a hunk of steel that deflects the heat.
The thicker the steel the better from what I have seen, my baffle and plate are 1/4 inch but I know guys using 3/8 for the baffle but thats darn heavy to work with.
One guy I know had good success just getting a 1/4" thick piece of 6" x 6" angle iron, he just cut it at 45 degrees or so on the verticle ends and at the right width to just sit on the sides of the pit on the horizontal.
