It is going to be pretty close to impossible to modify a cast front to the point of an airtight arch. An airtight has a locking system and the arch is pressured. For reference, I blow 760 cfm in my 2x8 arch. If it was airtight and pressured, it would blow ash out everywhere around the front. It also has air chambered around both sides of the door frame and the top and the door has 2 layers of 2600 degree 8lb ceramic blanket.

You can get most arches to the point where they won't blow out ash with some work and some sealing materials like rope gasket and ceramic blanket, but to get it to the point where it is airtight and pressurized is a totally different game.