The OP does not need to spend substantial monies to achieve a Design for a Tonearm Pod that will enable them to try out a alternative Tonearm and not butcher the Plinth of the TT, risking de-valuing it by potentially much more than the cost incurred to produce a Pod.
A Thick walled hollow metal billet with the selected diameter, can be machined with a very tight tolerance for the overall length dimension.
These can be found at an engineering shop as a waste item cut off.
The Side wall can have a Slot produced to allow a Cable routing.
The Base can be Tripod Configured Drilled and Tapped to receive a footer.
The Top Face can be drilled and tapped to receive the a Top Plate for the Tonearm.
Materials of choice and selected thickness can be selected to produce a Top Plate, the thickness should prove to be a tuneable material, and trialling a selected thickness can create a preferred interface.
The type of above task is the kind of undertaking a trainee will be given at a machine shop, and the charge will be reflective of this.
A Lead Shot mixed with a modellers clay such as Newplast can be used to fill the void and add mass, that should prove to be a tuneable material by adding or removing, again trialling a volume of mass, can create a preferred interface.
To somebody that is toying with the idea of trying out a New Tonearm and expressing a want to learn about lathe machining, an investigation into producing a Pod will be a good place to start, especially when a dialogue can be had with a professional metal worker
Note: As an advisory for a TT that is valued and has the pride of ownership attached that many do, keep Children, Grandchildren, Pets and Cleaners well away from the device, it will most likely all end in tears for the Kleos, and possibly even the Tonearm.