I have removed a Menards fireplace insert and am replacing it with a fisher insert (pre UL). The insert will be going into a exterior wall with 4'wx 18" deep insulated and sheet rocked chase to the roof approx. 10'ft. in height. I am using insulated stainless chimney with proper spacing from wall. Knowing these slammers have an air box around the back half how would I calculate side and rear clearances on this for combustibles? I had planned to put cement board on inside of chase but maybe steel is better. Also it seems that the UL listed inserts require 33" to combustibles from insert top to include framing members. (per owners manual) . Will a heat shield work for the 2x6 framing that is 18" above insert top? Or reframe opening in metal studs. I have the factory blower with this insert and will be using that to move heat around insert. Thank you gentleman in advance for your direction.