Tonearm base for Denon DK300 plinth?

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.

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.