How to mount a 60 amp 2 pole 240 volt non-fused switch
Those switches are designed for surface mount, the operator or handle needs to be open. In the US that Romex will need to be covered below 8’ not just the kiln but the existing wire to the light switch. 
Code allows for 1/2” sheetrock or plywood to protect the cables in the walls. 
I would use plywood to mount the switch especially if you will be regularly cycling the switch. 

The knockout in the back of the box can be used but you will want at least a 3/4” clamp I usually use 1” clamps with #6 4 wire cables because 3/4” is really tight and some brands do not fit 3/4”.  The clamp secures the wires and protects the insulation from the sharp edges of the knockout hole.