I got my shade tree in the other day and had the exact same issue as the OP. I did take a 1 1/2" piece of sched 40 pcv and cut it to the length of the shade tree assembly that goes over the seat pedestal.

I then cut about a 3/8" strip out of the pcv so that it would collapse and fit into the shadetree unit.

Then finally on the opposite side of the pcv from the cut out I drilled two 1/2" holes that lined up with the lockdown bolts that contact the pedestal. This way the "shim" is trapped between and by the bolts and will stay in place and properly aligned.

This made the shadetree fit the smaller diameter pedestal perfectly. You still need longer bolts but the force is distributed perfectly around the entire unit just like designed to do with the larger pedestal.

I also had to modify the lower section of my "beach umbrella" to be able to use the bushing but that was not hard to do either.

This is my first Millennium Marine product and looks to be well made and in my application I think will be hard to beat and still use an umbrella!

For the adjustability provided and what looks to be top shelf construction is very reasonably priced and I can see me buying more of their products in the future.