What are heat sinks closed on four sides with open ends called?

[JerryG]

I am trying to find a certain type of heat sink. I am looking for these heat sinks that are closed on four sides with open ends as shown in the picture. Can anybody tell me what these are called or where I can purchase them?

 

[AlephZero]

They look like extruded aluminum heatsiinks. Yours don't seem to have as many cooling fins as the usual designs. Are those rails just transferring the heat to the "real" heatsink mounted on top of them?

Do a google images search on "aluminum extrusion".

 

[yungman]

You have not shown how the inside looks like, it is very hard to judge.
Do you really need to have it like this. I think I know what the box is, it's some type of enclosure that has very short fins inside. If so, it's not the best of heat dissipation. I think if you use a real heat sink, you don't need to be this fancy to have a tube like box and fan.

You need to specify what is the power dissipation, what device you use to see what is the best heat sink.