Best way to cool external hardware

Hey Guys,

I’ve got a Timecapsule, external HD and a sling box in a cupboard/media centre.

They’re getting hot as hell despite the back of the unit have about a 4inch hole i - the HD is almost too hot to touch and I’m growing concerned about hardware failure and/or fire.

Was wondering if anyone had ay recommendations for cooling them? Maybe a low voltage/noise fan I can put in the cupboard? Heat sinks?

What do you use or recommend?