Raised Bed filling help

We’ve moved into a new house and we have a raised bed of sorts with nothing in it. We’ve put a palm in there in a large pot on some bricks to bring it flush with the top.

It’s about half metre square opening and goes down about 70cm before it’s clayish soil.

I’d love to have it like the final picture where it’s white stone with the palm popping out all flush.

Can anyone advise on the best way/cost effective way to either fill the hole to allow us to get the final picture or suggest some clever way to do it without having to fill it (eg cut a hole for the palm in a marine ply sheet and put the stones on that)

Ps I’ll probably replace the black edging with something nicer!