Here's a design I did. Feel free to give it a whirl.
I designed this rainwater harvesting nursery to deal with the tropical rains in Nicaragua. I never built it, despite a real need for it. I must admit, it was probably from my fear of making a mess of the whole thing.
However, if you look closely as the design, you can see that it is made to ensure that there is enough water at all times, but with an overflow for rainy season. I didn't want to waste this overflow though so I suggest putting soggy feet plants in the planter a the bottom; ginger, turmeric, lemongrass, maybe even water cress. The shelter is integral because seedlings wilt like crazy in the hot tropical sun.
If someone gives this a go or has any critiques of the design, I'd love to know. I'll be submitting it for my Permaculture Diploma in Applied Design so any help would be grand!