Tuning plates should be installed above the FB/CC opening... maybe 2-3 inches.... the first plate is usually tight against the FB..... Then spacing between the plates can vary.. 1/8 or so for the second plate and progressively increasing to the end of the CC.... leaving a reasonable gap at the last plate.... The plates can be adjusted by baking "pop open" biscuits on the food rack and adjusting the plates to adjust the heat.. OR, I have read where an infrared non contact therm can measure the plate temperature... don't know how good that works since you want to know the temperature a the cooking surface...
As far as plate size, probably 4" width or so.... 1/8" X 4" by XXX steel can be sheared to length to fit...
On some smokers, the first plate may have to be cut to fit the contour of the CC fit up area.....
# of plates.... 60" CC at 4" plates.. 15 plates would fill the CC... 13 would leave 8" if they were tight..