Hello , Funk. I don't know where you live so no info . on Scrap Metal Yards in your area. (Fix that by going to your Profile and filling in the location ...) A Scrap yard yields a world of good material for cheap.
For tuning plates of a carbon metal(Steel) and measure to the width of your Barrel interior ,then drop 3" to 4"below that level and get the width there...Cut about 6 to 8 plates to your width and from 6" to 5" , 4" and 3" (several and place them in the Barrel ,mark and weld an angle 1/4 X1/4 for your slides.
Now closing the FB to the Cooking chamber , use a flat plate wide enough to fit across the FB and long enough to fit to the level of the Plates.Bolt6 this into place.
The tuning plates can be of anything1/4" or so . Move them to target your heat.i.e.- build a fire , use a good therm. to track heat.(shoot for 250*F steady. Then test across the Grill, noting the hot and cold spots. Move the plates as needed to even the temps. across the barrel.
Hope this helps, PM< me if you have more Q&A.