While I was working out the details for how to mount the casters, I spent some time finishing up the rotating platform.  I made the fixed and sliding stops out of more of the salvaged ash already incorporated into the top.

After milling some pieces to match the thickness of the platform, I selected one for the sliding stop and routed a dado to accept an ash runner.

Because there's a specific side for each tool and a particular way I need the tools to rotate in/out of the cart, the runner needed to be attached to a specific edge of the platform.  I'm just screws to attach the stops - I want to be able to remove them if necessary to shave the platform's radial edges if the fit is too tight. 

Trimming the overhang.

I cut a set of the sliding stops to length - not quite half the width of the platform.

I got a box of these bicycle quick-release cam clamps on Amazon.  They don't seem too robust, but they should work for holding the sliding stops in place.  I removed the included threaded knob and substituted a dowel nut.

Holes for the dowel nut and the clamp shaft:

A slot in each of the sliding stops limits their travel.  Fully retracted, they'll be 1/16" in from the edge.  Fully extended, they'll stick out 1-1/2" inches.  (I added the extra extension just in case I decide to widen the seat where they'll rest.)

With the sliding stops mostly complete (I still want to glue on some kind of knob to make them easier to slide), I attached the fixed stop on the opposite edge and the platform was ready for action.