One of the secrets to achieving fused surface roots like on this Japanese maple (Acer palmatum) is planing (not to be confused with planning, which also helps).
If you plane the base of the trunk and the bottoms of some of the larger surface roots each time you transplant, the roots are induced to grow out in a horizontal direction and gradually fuse. This procedure, if done with care, does not compromise the strength of the tree.
A small, lightweight, simple carpenter’s plane does the trick.
The photos in this post are from Bonsai Today issue 17 (out of print).
Previous posts on nebari are: An Ingenious Technique parts one and two and In Search of the Perfect Nebari, parts two, three, four and five.