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.

I am guessing that you are either missing the tonearm mounting board entirely or that you have one cut out for a 9-inch tonearm.  This is a simple problem.  If you have a "wrong" tonearm mounting board, just measure it and have a new one made out of any material you desire.  If you have no armboard at all, then measure the rectangular space where the tonearm mount should fit the plinth. Then have the new arm board drilled to match the pivot to spindle distance of whatever tonearm you want to use.  (In this latter case, you'd also have to drill screw holes for fastening the new board to the plinth.) Keep in mind that it is possible a 12-inch arm will not fit.  I recommend re-creating the new tonearm mount out of aluminum or brass, not MDF as per the original.  This alone is a worthwhile upgrade to the ensemble.  A machinist who can help you is Colby Lamb in Oregon.  I believe he has a web presence.
Hi.Thanks for the answer.
Actually I don't have it... yet. Before buying one I want to be sure if I can get a custom tonearm base first. The web you make reference is, am I right? It does look nice. I'm in Europe, BTW, so I'll also be looking for a more local site (you know, shipping and taxes...).Is a specific non-magnetic alloy necessary?

Non-magnetic for sure, but after that it's your choice.  However, aluminum or brass are the two best obvious choices, if you choose to use metal.  Machinists like to work with them, because they are relatively soft. And "On-line Metals" is a good source.
Any competent local machinist can do it, it's not a difficult job. And you'll make a connection that may be useful in future, even if you don''t foresee the need now.

I find machine-shops fascinating. I needed a small part turned on a lathe; really simple. But while there I saw sheets of perforated steel, and asked him to make covers for tube monoblock amps. He liked doing them, it was a creative break from routine work, and he charged very little. They're beautiful, they protect the exposed tubes — and I never even thought of it before visiting that shop for something else.
Hi.Yes. I'm looking into it. I'm a bit more concerned about fitting. Sometimes it's quite difficult to find the required hole diameter for a tonearm. I know the DA-308 fits. I've seen pictures. The diameter for the DA-308 is 22 mm, so that's my reference for the moment.