Echoing others, I feel like keyholes in rows wouldn’t efficiently use your rectangular space here. You could still achieve the composting in place effect by making raised beds and putting a chicken wire column or trench in the center, and then adding scraps to that as you would a keyhole.
Since you don’t have access to a lot of brushwood, you could also still do the hugel idea by filling the bottom of the beds with wood chips if you can get your hands on some. Chip Drop did work for me but sounds like some have better luck contacting arborists directly. I had great success doing this with one of my beds, just have to add enough nitrogen to account for the wood chips. Great way to save on filling them! I did 3 beds “normal” hugel style with the logs and one with just wood chips and honestly much prefer the wood chip one. Could collect bagged leaves/yard waste around the neighborhood too.
Exciting work!
