This is how I boxed in the rather noisy 1.8 BMC on our motorised butty. It reduced the noise level from 86 to 58 decibels. I used the heavy quality accoustic deadening lining. It's possible to get lighter and cheaper versions, but they are not as effective.
The frame can be completely dismantled in case I should ever need full access to the engine. For example, the front end (nearest the camera) can be unbolted to get to the water pump, alternator belt etc.
The enclosure is lined out with accoustic lining.
The finished enclosure. The top and front can be quickly opened. The checkerplate is rubber, by the way.