Cocos nucifera has a very shallow root system. You could easily grow some of the smaller cultivars in 50cm of appropriate soil, as long as the soil extends sufficiently out from the tree.

A problem with the concrete base may be drainage. The tree requires good sharp drainage, and will need a good foothold to withstand coastal tropical storms.

If coconut production is not a requirement, there are many other palms which could work as well.

Consider making the raised beds a bit deeper, by going further up with elevated planting areas.