Artesania rack

While the rack should provide isolation, you might want to check the tubes in your phono preamp and see if you can identify and replace the one(s) that are so microphonic.

Tubes are microphonic. Depending on tube type and gain you may or may not ameliorate the problem by changing tubes until you find some less microphonic samples. Take a pencil with an eraser at one end, and gently tap each tube in your phono stage with the eraser end. That will tell you which ones of those tubes are responsible for the noise. You can then concentrate on finding quieter versions of that tube. Tube dampers are certainly in order and might be very effective to eliminate this problem. I don’t know what tube damper work best among the very many available on the market.

Would’t heat trapping be more of an issue when placing it on the bottom or even middle?

my tube amps have plenty of airflow and nothing above them to trap heat.