yes you can pull out the fan inside, and use a HDD cooler on top off box. Connect the HDD cooler with a scart plug, use pin 8 and 21 to get power to the fan.
You can mate yes the original 800s don't even have a fan. However they do get warm so its a good idea to have some cooling. You could wire it up as paraglider mentioned or use the usb sockets to power it.
A popular option to use is a laptop cooler powered from usb.
yes you can pull out the fan inside, and use a HDD cooler on top off box. Connect the HDD cooler with a scart plug, use pin 8 and 21 to get power to the fan.
By using pins 8 and 21 your fan will also switch of if the Dream is in standby.
You cannot start a timer in standby and have the fan running.
You'll have to switch the Dreambox on before the timer starts.
Pin 21 is not realy a pin. It's the metal connector (shell) of the scartcable.