tieing into existing fireplace with hearth model

I ordered a L.L. hearth model about 3 weeks ago and it will be arriving mid Nov. Since it starts getting cold here in ohio in october I'd like to get a jump on installing the piping. I removed the older woodburning insert and noticed a piece of sheetmetal they installed below the damper to keep smoke from enterring into the house. The hole in the sheetmetal is 6" and OVAL Shaped. I'm assuming they did this as the damper frame is above it and the pipe needs to be squeezed in an oval shape to go past the damper frame. My million dollar question is how do I add pipe and make it air tight to go 2' past the damper since the end of the pipe going through the sheetmetal and damper will be in an oval shape? I did clean and inspect my chimney and it's in great shape with an 8" clay flue...I also removed the piece of sheetmetal as I'm going to replace the mortor around it to assure it's airtight and am hopeful someone can assist me who has installed a hearth stove before... [b][/b]

Hi John,
The make 6" oval stainless steel flex pipe. You'll need to buy a small peice at the hearth shop.
Thanks for the purchase,
Jerry

Thanks jerry, that makes more sense because I could insert the piece of flex pipe through the oval opening and then I could attach the stove pipe to both ends and it would be a circular shape at the end of the flex pipe assuring a tight seal...I appreciate the input and advise.