Hi!, Danger005,
I have thought of doing this to a pond pump (pool pump and cartridge filter) and a water tank pump both located close to each other. I have some off-cuts of that cool-room material, aluminium/styrene/aluminium sandwich about 50mm thick. Thing's I have to consider is ventilation/cooling for the pump's but this should be no great problem.
This type of sound proofing is used by the place I used to work for, and they had some big pump's which made a racket when they had a bearing on the way out. But like most places, they would run them until they failed completely, and then replace it with another new one they would have on stand-by.
Seeing how you already have a cover made, just try to scrounge up some styrene sheet, try places that sell fridges (Bing lee and the like), as it is used for packing or a cool-room maker, they are willing to get rid of bits and pieces as it costs them too have it disposed of. Make sure you get the proper adhesive as some will melt the foam and you will end up with a gooey mess.
For vents the snap-in plastic type would do, but don't fit them in line set one up and then have a space and set the inside one off-set to the other one on the outside, if that makes sense!....Good luck!..