Use a big heatsink. I don't like fans (they are noisy).
But feel free to use a fan if you want
The heatsink should be rated at 0,25K or less /channel.
I think the Aleph 3 dissipates around 100W / channel, so the heatsink will operate at 25 degrees C. above ambient (0,25 * 100) if you are using a 0,25K heatsink.
Fans are ok if mounted correctly(isolated) and not driven to an audible speed. Use a 12 volt dc 4" diameter 5 blade fan and run it at about 6 to 7 volts and it won't be heard till you have your ear right on top of it. It does not take a trillion cfm to keep things cool, just some slight air motion in general to get the heatn away. There are also "Smart" fans that have built in thermostats and will automatically sense the temperature and adjust speed accordingly. In this case the heat is a pretty constant thing though and the smart part can be left out.
A high quality computer fan should do the trick. Some of the inexpensive ones have poor quality bearings. Often they'll be audible, and over time they may begin to squeak and eventually lock up. Every once in a while, I like to blow them out using a can of compressed air, and if you can access the bearing, I add a lube designed for delicate parts. Keeping it free of dust will keep it quiet and make the bearings last a lot longer.
Consider using smaller heatsinks, ( 1 per device, of suitable ratings of course ) and mount them to larger chunks of aluminium to add to the heat dissipation. I've tried it and it works just fine.