Why do you even need the tramming plate? If the spindle mounts had flanges either side, they could be mounted directly to the z stage plate. Tramming could be done by having slightly oversize bolt holes on one side. Might be less convenient and more time consuming, but once done, way more rigid, Also, if you're concerned with weight, the individual spindle mounts could be half the thickness of the original single mount. Or you could bore some holes or pockets to lighten things up.