Thanks! We have 600mm high beds and the bottom 150mm is the gravel water reservoir. That ratio seems to work very well.

I don't know how they'd handle freezing temperatures, though there is alot of thermal mass in them so they might survive if the temperature doesn't plummet too much or if they are in greenhouses.

I made ours into small, individual greenhouses last winter by bending some PVC pipe over them and covering that with plastic. They handled things well, but our temps don't get to freezing.

Soil mixes are trial and error. Im very lucky that the mixx from our local garden centre is perfect (but they dont give away their recipe lol). When you experiment, remember that the water must wick from the bottom to near the top , so either too much sand or too much compost will stop that happening. I would say, based on ovbservatuons that guess20% organic material would be the upper limit.

Let me know how you go!