Now I see the problem, which is maybe why Mark never designed a third stay for the e30 based car.
The e36 suspension geometry looks quite different...
Marlin demonstrator car Easter 2004 by
marlinpeter, on Flickr
If you bolted a small length of angle extrusion (~20x20mm) across the top u-bolt (on the wheel side) could the third stay then come of the top of the centre of the angle extrusion. At an angle but keeping it straight?