The plenum approach, as shown in these pictures, of course must be
very very strong indeed to withstand a back fire. Hense it becomes very
very heavy. Even 13 mm thick wall plate might not be strong enough.
Also the flat sections of plate have tremendous bending forces
on them tending to distort the plate as you have well
demonstrated here.
A round tube or pipe will withstand much higher pressures.

Paul C, Looks pretty wild! I hope I'm not covering old ground here but 
here goes. In Blown fuel cars in the US the sanctioning bodies often 
require what have come to be called "sneeze valves." They are open 
rectangular areas on the manifold sealed with a plate and gasket. 
springs hold the plate closed to just above normal blower pressure. 
In a backfire the plate is lifted off releasing the pressure safely. 
This eliminates some of the need for super thick intakes. It can also 
prevent one of those blower explosions that sends the huffer into a 
low earth orbit! Please excuse me if I'm covering old ground. I can't 
tell from the pictures if you already might be using this feature. Bill Jepson

 
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