I agree with the guy that said KEEP IT SIMPLE!!
Dont do any fancy knots that are time consuming and that you could get wrong.
Dont use a double or triple fishermans for the above reason,plus its a large knot, so when your pulling your ropes through, their is more of a chance they will get jammed.
A overhand knot with 12-18 inch tails is fine. If your worried back it up with another overhand knot. Simple, and can be tied in thirty seconds or so!!