I use a Targus Chillpad (I think that's the name) for my Inspiron 15.6", and with just the 2x 92mm fans it dropped temps between 12 and 24 degrees (Celsius). I keep my laptops' all very clean, no dust issues, it's just that they are that in need of better cooling!
I can't remember the name of it, but I have seen one that has multiple 100-120mm (can't recall) fans that you can easily move anywhere underneath the laptop to ensure they are directly cooling the hot areas.
Another thing to look for is a metal grill that the laptop sits on, which is worth a few degrees C vs plastic/rubber, but also increases the strength/rigidity of the unit significantly.
I would never purchase another one without a USB Hub built in, as you sacrifice a USB port just for the cooler. Some will take 1-2 but give you as many as six in return. A few have 2-4x USB as well as multi-card readers, too!
Lastly, do not forget ergonomics!
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