I bought a buck model 85 that has close clearance shields on it from a guy who told me it was a 20ZC. I didn't notice until I got it home that it was a model 85. I have a ventless gas insert right now, but I do have a wood frame chimney chase to the roof. I have plenty of room to install the model 85 but it is not zero clearance of course. The manual shows clearance of 12" on each side and 16" to back wall. The opening I have is a shade over 40" wide (could possibly make bigger)and the stove is 28" wide at the front. The depth and height will not be an issue, I don't think anyway. What I am hoping to do is line the sides,back, and top with durock then 1" spacers and sheet metal on top.I guess what I need to know is : Can this be done?Is the width using heat shields enough? How far off the top of the stove would I need the top of the box to be inside the chase? I have never dealt with a insert before only free standing stoves, so I need a little help.