Bypass ideas

Hi there. I have just spent my first winter with my new rocket mass heater and must say it has been transformative!
I did however have some (anticipated) problems with smoking with cold starts. Unfortunately a few of these are inevitable throughout winter.
So before next winter I intend to modify to fit a bypass. I have no welding skills to speak of so an off the shelf option would be very much preferred. I wondered does anybody know of a good product that they can recommend? Would a standard flue baffle/damper work or would it be best to completely choke off the exhaust directly to the flue when sending heat to the mass?  
I will include an incredibly technical picture to describe what I'm planning.
Thanks in advance

Hi Mike
Check these threads out.
I use a 4" blast gate as bypass
https://permies.com/t/155372/Installing-Bypass-bell
This one is similar to the high tech drawings you supplied.
https://permies.com/t/159700/Retrofitting-Piped-Mass-Bypass-Install

Near the end of this thread, you will see a different style bypass.
I modified a standard cast iron damper to work as a bypass.
https://permies.com/t/267527/Shorty-Core-bell

Here is another retrofit
https://permies.com/t/152857/Blast-gate-bypass

Thanks Thomas that's just what I needed!