Since you still need your shaker grate to work, and you have to block off ALL the air from coming through the blocked off grate, I used a 1/2" thick plate of steel that rested on either side of the grate, leaving enough room underneath for movement of the shaker. The plate sealed off the air against the back wall of the firebox.
I then made a framework to hold a stack of firebrick, the framework had a 'foot' or base that rested on the 1/2" plate.. This framework formed a new back wall for the firebox.. In my boiler [Big Bertha] I reduced a 50" long firebox down to 24-26". This back wall was two firebrick high, allowing an 18" deep fire at the back of the fire..
Here is a photo of my firebrick back wall..