Hi,
only in idealistic case ... where the "heatsink" has
* zero capacitance against 0V, 300V and earth GND
* infinite resistance / impedance to 0V and 3000V
This is not realistic.
We don´t know whether the voltage is DC, and if AC what frequency and waveform.
In case of DC.. when a spark happenss (due to ESD for example) the spark may continue and cause a danger for life and even may cause fire.
... nor de we know heatsink sizes, protecticve earth availability, environment, accessability, casing, dirt/dust ... ... and so on ... this all needs to be considered to find a useful and safe solution.
***
If you can´t find suitable resistors .. then I´d mount these resistors on individual (bigger) aluminum or copper plates .... and with the use of suitbale insulating material ..on the bigger heatsink.
For sure this increases overall capacitance to the heatsink .. but due to the wider heat transfer area it may compensate for the increased thermal resistance caused by the barrier.
Found some information online (No guarantee)
If true then a 125um thick capton foil can widthstand more than 15kV. For sure one would not use it for 15kV, one needs to add margin.
Klaus