DIY OAK thru fireplace cleanout

I'm thinking of installing an OAK for my Whitfield Profile 30 insert, via the clean-out chute at the back of my masonry fireplace. The clean-out is covered by a fitted 3"x6" metal plate inset into the floor of the firebox, which I was going to remove and replace with a larger piece of sheet metal. I thought I would cut a round hole in this piece of sheet metal, just large enough the fit my air supply pipe thru. On the upper end of this pipe I was going to attach a 90 degree elbow that would allow connection to the combustion air inlet at the back of my insert. The bottom end of the vertical pipe I figure can just let open into the cavity underneath. To allow outside to enter that area, I'm thinking I can drill several small holes in the cover plate that attaches to the outside of the brick chimney.

Has anyone tried anything like this before? Can anyone see any problems?

One other thing. I've been looking for the right size of metal pipe to use for the air supply and can't seem to find anything small enough - the air inlet at the back of my insert is only 1.5". Could I use ABS plumbing pipe for this, or that a no no? It doesn't seem like that area would even get warm, let alone hot, but I seem to recall reading somewhere that an OAK must be metal.

Hmmm, interesting. I wondered about how much outside air the stove would actually be able to draw in, with those 2 large holes in the sides of the intake pipe, and a steel plate partially (80% or so) blocking the pipe between the holes. Why did they design it like that anyway? I've read that it helps the combustion to mix in some warmer air, but the Profile's design seems to heavily favour inside air. What about if I were to block one of those holes with some foil tape? I considered removing that block-off plate but it seems to be welded solidly in place.

OK I see what you mean. I guess I'll re-think the OAK. Maybe if I just drilled some holes in the 2 clean-out access plates it would allow some outside air into the back of the fireplace, which the stove would then draw in rather than pulling warm room air in around the front. Thanks Scott.