What type of cord would you use?

I had some left over sisal that I used to make my first one, and it turned out pretty well but I wonder about the strength of it. Particularly if it were used for something like fishing instead of against small game (birds, squirrels, rats etc).

So for my second (and current) I'm using mason line, which is (as you can imagine) a bit of a headache. It does work after a fashion, but it has issues around the edges and then knots slip making large holes in it. The knots also have a habit of coming loose just in general.

Thoughts?

Because of its slippery nature synthetic twine is a difficult to make nets with.  Instead use cotton, sisal, jute or other natural twine.  If you want the strength of nylon use the double sheet bend or switch to a bonded nylon or spun nylon.
 Check out the "Nets and Net Making Section of the Pineapple Knot forum.  Over a dozen netting related topics including instructions, hints and tips including the double sheet bend.
http://pineapple.myfunforum.org/index.php?c=7
 Just below the netting section (Knotical Resources) is a list of sources for net making cord and needles.  Good luck.  Tony

If you made it for fishing, you may want to put loops in the corners.  I know the holes in the net would work for this, but this way you have a designated place to hook another line to.  If you make them connect up like a classic parachute (where the four corner lines attach to one central line), you could place it in the center of a stream or drag it behind a boat.