Here's a large vase that I made into a planter without drilling any holes.

I kind of created an aquifer at the bottom so the excess water would drain away from the roots ... Rocks, then pea gravel, then crushed seashells ... It helps having a clear container because I can see how much water is at the bottom. One idea I found after making mine was adding charcoal ... Mine can get a little murky looking at the bottom if I accidentally overwater, and I'm curious if the charcoal would help with that. I used to have a decorative wooden box that I drilled holes in ... I lined the inside with a piece of plastic and didn't let it sit in water, but mine was inside for houseplants. For metal containers, you could try using some kind of sealer on the inside or putting in a plastic liner. Sometimes you have to experiment a little and be willing to repot if it doesn't work! =)