You can just screw in small self-tappers into the number of holes you want blocked to get the heat down....unless the inner metal ring thing you are considering is part of the setup, like you are setting a certain pot or smoke container on it and its part of the rest or stand-off?
I'm in the hot part of California with a fully insulated 44"
Masterbuilt and I don't find that the 40K is too big. I can go below 225 even on a 100 degree day. When I need to go real low, like 150, I just turn my pilot light on. My air flow is pretty heavily modified so maybe my smoker is staying cooler than a regular 44"
Masterbuilt? Anyway, my thought is that you may be cutting it down too far but admittedly I have no idea what you are trying to accomplish and how big your smoke chamber is.
The bug guard/venturi adjuster can be turned and made smaller, so that you can compensate for you blocking some of the orifices. If it's not enough, I'm guessing you could put a bushing in the pipe or use a reducer in the stand off pipe to get the air flow down. Just putting screws in will answer a lot of your questions because you can do it on the bench top and give it a thorough test before committing to a design or build.
I think I'm going with the 80K on my next cabinet, I want to go up near 500 degrees occasionally and I'm hoping it will get me there. I'm a little concerned that it might make the 225 hard to hold in the summer but I'll block some holes if that's how it works out.
On a side note.......anybody having heat issues with their MB, I've had really good luck going with this burner. The MB one is absolute garbage and appears to have been designed by a liability attorney. Same with their smoke system. Putting the Tejas 40K in solved a lot of my issues and actually made the flame adjustable. With it and the optional pilot light, I can go from ambient temp up to 400 and everything in between and hold it wherever. It also seems to be pretty darn fuel efficient although I haven't really quantified those numbers yet. I'm running tubes for smoke so now heat and smoke are separate entities. It's nice having heat and smoke separate; I can cold smoke, go no smoke and using it as a warming oven, I can hold 225 and go with 2 tubes if I want a lot of smoke, etc etc
Hope that helps-Norcal911