I just patched a 4cm diameter hole in a plastic water tank so I could use it at the lottie. I got some plastic sheeting (which was recycled from work - they get it in x-ray film packets) and went to the DIY for some glue. After reading just about every tube going, I settled on something called 'serious glue' which doesn't have any solvent warnings on and doesn't pong, is UV stable, waterproof and temperature stable over a huge range. Slapped the patch over the top and then stuck loads of weights on top so it had good contact (since the water tank was distorted due to the pipe that had been attached).
When it was dry (about a day), I fed more glue round the edge of the hole from the other side, to make sure it was sealed, then clapped another patch on the other side.
Not a leak in sight. So patched a hole in another tank too. So far so good.
moonbells