If you need more channels look here: http://www.thingiverse.com/thing:163580
Simple MOSFET board with galvanic isolation. This could be usefull if you want the power supply for the heatbed to be isolated from the printer power supply.
NB: This has not been tested yet. Be sure to check the datasheet of the MOSFET. This may not work as expected at high frequencies (to be done in future designs).
This board is a general purpose board. You have to select the values you need and you may not need all the components.