One way to rig the tack of a lateen sail...
 Tie a knot at the tack of the sail, at the forward end of the spar, carve a small notch, slice up a small piece of flexible plastic, about 1 cm round, with a hole, drilled or bored in the centre small enough that the spar can squeeze into (with a little force).
Tie a length of line near the end of the boom, a notch there will hold the knot in place. Run this line and the corner (tack) of the sail through the bit of plastic, then push in the spar, the plastic will click in place when it reaches the notch in the spar, the end of the line is then drawn through, to draw the boom and spar close together.
A stopper knot will keep the line from slipping through, if you use a length of line long enough,
you can tie it to the stem, this will prevent the sail from swinging beyond a beam reach.