Making a solid state switch.

[hyudryu]

Hi, for my next project, I wish to make a MOSFET for a quarter of the price. I saw this http://www.infectedarmory.com/AEG_AB_L.html
and I see that they are selling it at 25$. I bet the parts inside like the power transistor, only costs around 7$. The specs on the store sold MOSFET are 300A+, 18V.
I have experience with soldering, and circuit designing so I believe I can make the exact same thing for a fraction of the price. There is a local electronics wholesale supply store near my house where things are very cheap. So for my project, I would like to make a solid state switch that can support up to 20V, and preferably, 300A+. I doubt an airsoft battery can supply that much current, but better to be safe. I have a full understanding on how transistors and thyristors work, but I have never bought any that is small and can support that high of a current. Any suggestions on which kind of solid state switch I should get?

 

[Resqueline]

One definition of Solid State Switches (or Relays) is that the output is galvanically isolated from the input signal. Is that a property you'll need for your project?
Another definition one may make is that their switching speed is not neccessarily very fast. What kind of switching speed do you think you'll need?
If you need isolation with limited switching speed you could for example use opto-isolated photovoltaic MOSFET drivers, such as these.