Hello. I would like to get a few opinions on my wood insert installation. I will be installing a Pacific Energy Neo 2.5 into my current heatform fireplace with a yet undecided liner. I meet all the minimum clearances for the firebox except for the height req. at the rear , of 22". It is because the heatform slopes up at that point. Would it be OK to cut that portion out and either leave open and then insulate around or should I weld a straight piece of steel up from the cut to enclose the space.