Hi – and welcome to Beoworld!
The bog standard 9V adaptor should work. I’ve heard 9V 2A been recommended (the original as pictured, appears to be rated at 500mA). The plug should be center positive, and physically it should be slim as otherwise it won’t fit into the crevice.
I believe there was no circuitry inside the stand, so you should be able to verify if the 9V is going to the contacts on the top of the stand.