Brian, Boon grew mine in a slightly larger and deeper bonsai pot. Several of his students use colander a or pond baskets. Some tried the "double colander" and later wished they hadn't. Singles work just fine.
You might want to try "potting it deep", and see if it will throw out roots off the Zuisho. Boon tells me that every Zuisho and Kokonoe graft on JBP stock is doomed. The growth rate differences of the two pines eventually cause the graft Union to fail. But, both Zuisho and Kokonoe will ground layer. Then, you can eventually remove the JBP roots entirely.
He also says that most all the Zuisho have grafted on branches.
One good characteristic that Zuisho has over Kokonoe is the branches of Zuisho tend to stay slim, whereas the branches of Kokonoe tend to fatten up. Having slim branches makes the trunk look fatter.
The one I have now is the only Zuisho in Boon's garden. And, even then, Boon thought it was a Kokonoe! When Boon bought it in Japan, it was sold to him as a Kokonoe. It was Diasaku Nomoto who identified it as Zuisho when he was working on it last January. They're very similar. (I'm going with Diasaku says, he specializes in JWP).
From the look of yours, with that trunk, it looks like it's going to be a Formal Upright. With that in mind, you might want to start grafting on branches.