The keyhole design originates in permaculture. The basic type is the simple keyhole shape for accessibility. Toby Hemenway discusses it in his book Gaia's Garden (about half of this book is useless for South Florida, but the rest is great!)
An international non-governmental organization working in Africa devised this type of keyhole garden pictured below, with a composting basket in the middle, and it usually taller than this. See here: www.cowfiles.com
The African keyhole concept uses local materials of course, with a basket of reeds in the center. Here in South Florida, we like to use 4 lengths of rebar driven into the ground, surrounded by hardware cloth. A lid can be made from hardware cloth or other material if critters are a concern. I added a piece of cut reed fence just to remind me of the African style. This South Florida-style keyhole garden is a bit expensive due to the stacking blocks on the perimeter. We can install one for you, with organic fertilizer, soil, and plants or seeds appropriate for the season, ready to grow for $695 as shown here, soil depth approx. 12 inches. Add two more courses of stones for $895, making soil depth approximately 24 inches. Profits go to our urban garden projects. Contact us.