We dont use one as I reckon you can only tell if the headbearings are notchy etc if the forks are hanging.
Or have you tried the large plastic lid from a Big tub of Swarfega or similar? I use these as turntables to turh bikes around on their centre stands.They may work as a swivel plate too?
does any one know where i can get a second hand or cheep new wheel swivel plate for motorcycles, as there not required for the test now we never got one with our set up, but it does make it easyer to teat bearings with it