FWIW: I'm in a similar situation. I'm "very new" at this - last week I got a 6" Craftsman (Atlas) that came with a lantern post, and several holders very similar to what you have. But my post was missing the "rocker" (aka 'boat') and the "rocker washer". But I found that the larger-ish holders can be mounted in the post *without* the rocker parts. Also (as someone mentioned above) a 3/8" cutter can be mounted without a holder at all.
SO, the first thing I made was a rocker washer, using a simple square washer that is 3/16" thick and has a 3/4" hole to match my post. It was a "sub-optimal" setup, but it worked, and I learned a couple of things in the process. I made the rocker from 3/8" square bar stock, using a bench grinder.
So now I have something that holds the smaller holders nicely.
I had another of those square washers, so I used it to make an "angled washer" (one edge thicker than the other). It allows flexibility when using the larger holders in the lantern (which was intended for the smaller holders).
For now, I'm sticking with the lantern post. I enjoy using old tools the way they were originally made, rather than "modernizing". I'm in no hurry to get things done. That said, with my old wood lathe, I *did* buy a $40 set of modern carbide-tipped tools to go with the traditional tools. The new-fangled tools sure are nice for some things...
Pete