in order for the stoker to function properly, you need a hearth; ideally you would have a big boiler and not have to pour the hearth all the way to the boilers edge and could set a large ash can under it, thus allowing you to rake the ashes off the hearth into the can and remove it once it's cooled. Most boilers will not have anywhere near that much room and thus you must pour the hearth all the way to the boiler wall. you need a good bed of ash on the hearth to insulate the fuel bed, usually about 4" of ash with regular removal of any clinker that forms (once/twice per day, sometimes less). I wouldn't bother preventing the hearth from reaching the tuyeres, the expansion and contraction of the tuyeres will loosen the concrete such that it will not have a tight fit. those tuyeres should last years and years without damage, if they are damaged quickly, you are firing very improperly: using too low of a feed rate and too much air, or allowing the hopper to run out of coal. this is why when you set the air controll, the fire should produce no visable smoke at full fire, but just. do not allow too much air or the fire will burn low in the pot and you will burn the retort/tuyeres and, in some cases, the screw.