I would design in some more clearance than 3-4mm from crucible to furnace ceiling. I would probably try an A2 or A3 until you think the unit is proven worthy of a chopped A4. PS If I HAD to...I would chop one on a wet tile saw, shimming the small end while cutting the big end. Dry it off, let it air dry, then dry-fire it slowly to remove any moisture from cutting. That worked well for my plinth.