I started a little half-barrel pond in a hard liner about a week ago and finally found a half-barrel to put it in. Unfortunately it's not a good fit - the liner is a little too narrow and tall.

The barrel is pretty old and even when swollen up it's not watertight. I've tried lining it with soft pond liner, but that bunches up so much that it seems to fill a quarter of the barrel. If I cut the liner into pieces to avoid bunching, I can't figure out how to seal it properly again. I also wanted to avoid having liner sticking up out of the water if possible.

I read that you can seal the barrel with an asphalt-based sealant. Went to two hardware stores tonight and no one had heard of this. One guy said to use Drylok, but it looks like that's made for concrete, not wood.

Would polyurethane work? I think it'd need many coats, and I'm not sure how long it'd last. I'm imagining smearing the inside of the barrel with something like tar, but of course I don't want to poison the fish.

I'm getting really frustrated. Does anyone have a idea how to seal this thing?