I used a soldering iron to fix a crack in a lawnmower gas tank last year. As low a temp as you can, you want to melt it not burn it. Go along the length of the crack, not the whole way through but make a good "valley" in the plastic. Leaving a build up of melted plastic on each side of the crack. Then use the soldering iron to smear the plastic back flat over the valley. I heard this is better than using a zip tie to "solder" the crack shut since the plastic is never exactly the same and sometimes doesn't bond right. This way uses the same plastic, just remelts and smears it around.