Top plate is 5” circle cut into 3/4” pine, screwed onto the wedge itself which has its diagonal at a 42° angle (latitude of Boston, MA)

For easy assembly, I used 1/4 inch threaded rod and a knurled knob to attach through the wedge into the original plate on the base of the SLT mount. The original mounting screw on the tripod connects to a captive nut on the bottom of the wedge.

A vixen-style rail made from aluminum attaches via the tripod screw on the camera, and seems to hold the Nikon well through all angles in the SLT chuck.

Now all I need is some clear skies, to see if the SLT will polar align and track properly in EQ mode.