I don't recommend placing inside any cabinetry if possible. It will effect air flow. However, if you need to. Daves suggestion of the Laptop cooler USB or Not will be a great to have. I have one for my router and it's not in a cabinet. ISP modems generate a lot of heat as well. If you can place in a room thats not a sleeping area, keep both units out in the open on a flat non-metallic surface.
Let us know how it goes...