This is a scratch design with the general idea taken from Sjouke. Thanks.
I've designed two versions. One with the mosfet in horizontal position and the other in vertical position.
Remove the two lower screws and spacer under the mainboard and replace with your chosen mosfet holder.

You will need 4 x M3x8 size screws to mount the mosfet.

I recommend using the vertical version in conjunction with my "Anet V1.0 Mainboard Cover with 40mm Fan" http://www.thingiverse.com/thing:1803120
By doing so the mosfet heat sink will also be cooled by the fan.

By request I've added a cover to protect the mosfet.
In this case you will need 4 x M3x45 size screws.
Use a 3mm drill to completely open the grid holes if you feel the need.

I also made a cover where the heat sink protrudes out the top.
You will need 4 x M3x25 for this version.

Please view the pictures for more details.

I hope these holders are useful to others.

If you made one please also post a picture of your setup with the cover. Thanks.

Leo

The connector for the "Control In" is a JST XH 2pin.

I will post the holder mounted to the printer as soon as I can. But I'm sure you know where it goes and how it's connected.

I've added a twin mosfet holder to the files section.

I've added a mosfet holder to the files section for the mosfet made by www.fysetc.com.