In constructing a kitchen-waste digester at home, I use a 50 Litre HDPE drum. The base of the drum is holed with a plug fitted to allow drainage when necessary.
The top has two openings - one for inlet, the other to act as outlet for the generated fluid CO2/CH4.
The drum is to be used filled to about 2/3rds lying on it's side. Therefore any leakage at the drain will be immediately visible. The same applies to the inlet pipe - it too shall be filled with water.
A heated iron nail was used to make the outlet hole. Then a ball-point pen was inserted into it; a very snug fit. I then melted a silicon glue-stick to attempt to seal any leaks at this point.
The problem at hand is to determine whether the outlet is air-tight. It is possible to dunk this container in water and test for bubbles; but the same test will not be possible with a larger container (300 litres) to be used if this experiment is successful.