I have 6 of the plastic adapter plates, these are the ones from the VW Tourag. Part #077905390
I modified these ones and drilled the silly plastic knobs out so I could use regular bolts.

These leave a tiny gap on 2 corners that you might want to fill with RTV sealant. You can find photos of what I mean by a little google search.
These are about $5/piece. Give me an offer and I'll probably take it.