Blast gate bypass

My shop batch box dragon has always been a little finicky starting up when cold so I decided yesterday to install a bypass to see if it would help get the draft going a bit easier.
After a bunch of fiddling with different ideas, I settled on a blast gate design. Here is a typical picture of what they look like:
Pull the handle out to open, push in to close the hole. Easy-peasy.

source
I didn't want to wait for many weeks to get one in the mail so I decided to fabricate one using an old microwave casing for the metal and securing it together with a pop rivet gun.

This 4" bypass hole is about 20" above the existing chimney entrance just above floor level of my bench bell.
So far the slider works great and feels solid, providing a really good seal.
The two burns I've used it have made starting easier.

My shop dragon has two double stacked barrels so it does still take a minute or so to build up enough hot gasses to get a draft really going even with the bypass open, but much simpler than the method I was using before - making a little fire under the exhaust entrance within the bench through a cleanout port.

Here are a few pictures showing my progress. I've since shortened the handle on the gate and made a 90 degree bend as a handle (just like in the above photo). All is now sealed up with cob also.

Wow Great Job Gerry!
Looks like you only had a few inches clearance to get in and cut access hole's  into the stack and the manifold.
Hard to tell from the photo's what your connection piece is between the pipe and manifold.  
How have you sealed it ?
We definitely need more photo's!  Or if its all cobbed over already, then finish photo with explanation please.

Hey Thomas,   Cutting out the hole through the cob was easy as I started from the inside and worked towards the exhaust pipe. Since my heat riser is made in two sections, by removing the top section gave me even more room to work without accidently hitting it. The hard part though was cutting the hole in the vertical exhaust pipe. This is where one of those very small battery operated reciprocating saws with a metal blade would have come in handy! (Note to Santa).

Without such equipment, I first drilled a hole and then enlarged it enough to get a hacksaw blade (without the handle) in there to cut slits to fold back. A lot of bending of the body in an odd angle and many cuss words later, it was done. A bit crude but its amazing how much cob covers up all your sins!
So to answer your question, the connecting piece and sealing is all done with cob.

Here is a fancy animation of it complete and all cobbed in opening and closing.
BTW....That green piece of metal on the top is the lower portion of a heat shield.