drainage design for winter outdoors container

[bower]

I've been growing out garlic bulbils to increase my stock - planted in containers in october, mulched and wintered outdoors. I have one large flat that works really well, because it drains really well. Last year I added a couple of small tubs with drill holes in the bottom, shown below.
These tubs didn't drain as well as I expected.
We had heavy rains at several times when the soil was frozen, and ended up with standing water on top of the soil in the less than optimally drained containers.

So, looking at the containers, I can see that I could improve the drainage by drilling holes in the corners and closer to the edge on the sides... Also there's the option of putting some holes in the sides near the bottom.

I'm looking for advice, what is the best design to drain these tubs or other square or rectangular containers, so the rain will run out even if the soil is frozen...