I would probably just buy Acoustic Panel foam off Amazon or from a local sound store and line a thick box with it. The foam probably won't make it totally silent, that's not how it really works but inside of a box and I expect you'd notice a huge difference. And it's a cheaper route than going full 100% sound proof, though they do make sound proof insulation. Scrap wood or wood in general would work, if it's overly thick cardboard it might do it too. My only problem with that is, and maybe someone has a decent idea to solve it, is taking the heat out. If heat would be an issue at all, not sure.
My first idea if you did have heat would be to create your pump's main case, then cut a ventilation hole up top that runs into another smallbox (or just create this as the lid) on top, that serpentines the air out around foam covered baffles. If that makes sense. If you want to get real fancy computer fans could be used to assist in pushing or pulling air which would be quieter than a pump drone.
Just brainstorming here what I would do.