I guess you could also fill that space on top with an isolation base. Like the Auralex or some cheaper alternative?

But better and cheapter to get this 2X1 Kallax, stand it on end, and put your TT on top. You could still use the Auralex, especially if you have hardwood floors. I found that some vibration isolation made a difference

Sound Addicted VibeBoss P10 -... https://www.amazon.com/dp/B08RNJPX7L?ref=ppx_pop_mob_ap_share

Sound Addicted Isolation Pads for... https://www.amazon.com/dp/B08D9N9W3R?ref=ppx_pop_mob_ap_share