Tonearm base for Denon DK300 plinth?

Hi.Does anybody know where to find a tonearm base for the DK300 plinth? The plinths I see have what looks like a tonearm for a 9'' arm (or sometimes a 9'' tonearm already mounted), so, obviously if you want to use a 10 or 12'' one you need a different base... that can't be found anywhere separately.

Thanks.