You could fit a Waters canister inside you're tailpipe if it is for an MOT 
Might work for a while 

MOT emissions are a different matter, actually. They don't measure carbon dioxide (CO2) emissions at an MOT, only carbon monoxide (CO) and Hydrocarbons (HxCx).
These are essentially traces of unburnt and incompletely burnt fuel, and the emissions which are poisonous and do real damage, unlike the imaginary damage to the environment that CO2 does. A car equipped with a catalytic converter will clean these up effectively, so the MOT is really just verifying that something hasn't gone wrong with the cat.
An MOT test also doesn't measure the volume of gas, only the proportion of gases in the exhaust gas mix. So, an MOT test doesn't tell you anything about total emissions. You could hook a 1.1 Fiesta and a V12 Jag to a MOT tester and the readings would look the same. The Jag would be pumping out several times more volume of gases but we're not measuring that.
Kevin