Do what I did in my offset and use fire brick as tuning plates. A twofer so to speak.
I arranged the bricks across the bottom of the cooking chamber and have the usual deflector plate mod in place to force all the heat and smoke from the firebox to go up under the brick. I varied the spacing between the bricks from narrow to wide across the pit from end to end. Narrow gaps at the hot end and wider gaps as they approach the flue.
The brick adds much needed thermal mass to the el cheapo offset and varied spacing results in cooking temps are practically dead even from end to end like a tuning plate would. Especially as I always lay a piece of foil on top of the bricks and under the food on the grill to catch drippings which makes clean up a simple and easy proposition.
I know this is an old post but I really like your idea of using firebrick as a cheap alternative to tuning plates. I'm going to give it a shot as I picked up 12 firebrick at $1.35 each at a local masonary place. I'll wrap in heavy foil and stagger in the cook chamber and then maybe a couple in the firebox. It's cold up here in North Idaho (so I definately need the thermal mass) but I have a few days off and want to BBQ! I'll let you know how it works.