plant root protectors

I have a problem : under wild trees it is quite impossible to grow flowers. The wild tree roots are too aggressive to let anything else grow.

I had a solution : a container under the tree provided the controlled environment the flowers needed, but the container is both elevated and offers restricted area.

I have a question : is there a cheap way to build boards (say, 15" x 25") so I dig a hole, arrange these lying next to each other, so they line the walls of a hole as big as I choose each time, and fill this up with soil and compost and plant in it what I like ??? I think they could be made out of concrete, but I don't know if there could be a faster method, or cheaper material ???

Any ideas would be appreciated

Dimitri