Shun
Mame
Here is a Zelkova I screwed to a board;
View attachment 173060
The trunk is attached to the board by a screw coming up from below and into the base of the trunk. The green screws are positioning the roots. They sit beside the roots, not thru it.
View attachment 173061
As potted. A layer of bonsai soil first. Then the board, then an inch or so of soil. Then chopped spaghnum moss over the top.
What you did with your zelkova is very similar to what I have done with my kotohime except the roots adjustment with screws because the roots i had were very fine and fragile! Your tree already have a Nice base! How long have you been training the nebari with this technic?