How to make a tight seal between two metal components?

I would improve the fit between the lid and the hopper and increase the weight of the lid so that it is held tighter and has minimal gap to allow any escaping powder. also, if the design of the lid and hopper can be changed then i would incorporate a channel around the top of the hopper that the lid fits on to so that the powder has a longer route to take before it can escape. This is often done with drain covers and rodding holes to reduce leakage.