Yes, it's the DeRose plate that's the problem here, not the Matthews "Hi Hat (up for debate at a later time)". I've come across a few operators who have the same beasts. There's no internal plate / ring to stop the slippage. Would it be worth welding on a machined washer of sorts to accommodate? A sort of retaining ring.
It may be frustrating as there's no "meat" for your plate to grab on the Hi Hat - just an outer 1/4" (also try putting Mitchell risers on a set of Ronford legs). You'll have more success on a differently machined Mitchell plate (ie on a dolly). I'm surprised there's still a keyway locating pin as they are generally removed for better socket block placement.
On a side note, I've noticed that Arri increased the same ring on the ArriHead 2 so much so that my Cardellini Headlocks won't fit.