I got an older Regency r14 from a neighbor and it needs an offset box so the output can make it straight into the chimney. I was wondering if there was any advise on using an offset box? Particularly if I definitely need any additional parts to install it. I read something about needing high heat silicone? The problem is, the clearance into the fireplace is not large enough to install the box then slide it in, I would have to attach it to the liner and then slip it into the insert. I would appreciate any information. Thanks.