Thanks Al, I don't think I'd mind turning it into a shohin. If this tree was yours why would you d?
I have no idea what a "d" is but if this were mine I might be tempted to transform it to shohin due to the large varience in the transition of main trunk to upper trunk. It is abrupt and will always be so.
While the canopy of this tree looks nice in leaf, the tree should be appreciated more in Winter after the leaves have fallen. If the goal is to keep the height it has now, then it will definately need to be grown out in either a box or the ground. It will never thicken in a pot. Keeping low branches cut back and allowing free reign on the upper branches could fatten up that section and help with the overall taper of the trunk.
Either way (shohin or larger )it is worth doing the time as trunks like this are harder to come by and will increase in price as availibility goes down.
Work with the tree for a year or two, a shohin can always be a plan "D" if needed.