I'll try to answer this for you, but I hope Steve can comment too.

You can place the ports somewhere else but it's better to keep them in a corner, I think it's part of the tuning.

If you put four ports in it, you have to make them smaller in diameter otherwise the tuning changes.
4 ports instead of two : multiply the diameter of the original port by 0,58071
But this might give some turbulence at high volume.

Or you can make the ports longer for higher SPL.
4 ports would have to be 6,1333... longer then what the plan says for two .ports (being the thickness of the wood used)

Good luck and let us know how it turned out please.

Bart