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I also found 45mm x 75mm x 20mm (fin height)
This one is perfect , i don´t found this size on local shops but i made similar ones on my own 
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I also bought a 120mm fan and a step-down adapter to fit the case. I may also add 40mm sq. fans on the large heatsinks, and plug them into the unused 2-pin power socket on the gridseed.
Good idea! Nice!It´s interesting to do some tunning on this parts , but now they "eat" more money , some heatsinks here some resistors there .... but take this as learning lesson , i don´t know if the additional 1 - 2 MHS will cover the costs and the time spend ever.
Not only "just for fun" i hope...It will frie your device in fev seconds so add fev thermal switches which will stop device before it is fried.