Square Peg In Round Hole?

I bought the wrong wall thimble to replace the original, but it is the wrong type. I want to try to "Jury Rig" a way to use it & am looking for ideas. Here is what I bought: http://heating-and-cooling.hardwarestor ... 38259.aspx

But I don't have access to the interior side of the block chimney it will be going into. It comes in two parts (to make it's depth adjustable) & I somehow have to get the large flange through the (roughly) 8" hole in the wall. I can always cut down the flange & bend it through the hole & keep hold of it with picture hanger wire so that I can keep tension on the exterior half while I slip the interior half on from inside, & then sheet metal screw the two halves together when I get the right depth. Conversely, I could just use the interior half with a little stove pipe attached to make sure that it exhausts into the center of the exterior chimney.
Is this making any sense to anyone?....Any other ideas on how I could salvage the interior flange so that it will seat against the inside of the chimney?

I'm not an expert and am not sure about fire codes but your second idea sounds like it's the easiest. Trying to bend the flange back inside a chimney sounds like it would be tough to do. Other than keeping it in place I don't see the need for the flange inside the chimney.

That thing is only to insulate your wood/plasterboard wall from the stovepipes heat. It does not attatch to the chimney itself in anyway. You should have a tile thimble in the chimney that the stovepipe goes into. You need to cut a square hole about 2" smaller than that thing at it's widest, then install blocking around the holes interior. The inside flange can then be inserted at a 45* angle through the hole and the unit can then be assembled. Do not use it for attatching stovepipe to the chimney.