[Closed] Centerlock Ventidiscs & 20mm hubs - compatible?

My front wheel has a Shimano XTR M976 20mm bolt-thru hub.

I've just bought a set of centerlock ventidiscs and had a go at fitting it all together. The hub has a different lockring (due to the larger aperture in the middle) which tightens using a Shimano HT2 BB tool. However, due to the fins on the rotor sticking out, the lockring sits flush with the fins and so it is almost impossible to get the BB tool onto the lockring to tighten it.

So, is it impossible to use ventidiscs with 20mm bolt-thru hubs on a centerlock set-up? Is there some other tool to tighten the lockring? Or is there a Magura-specific lockring?