I've tried to create something like this:

I made frame and simple box on top of the frame but had a problem of creating natural wooden edges with sculpting but run into this:
Can somebody show how to do that with sculpting or deform modifier (as I remember there was quite useful box deform modifier in 3ds Max)?
PS I tried, Triangulate, Subdivide Surface and Dynamic Topology. Just cant understand what I am doing wrong.
