That fixture is part of a draftsman’s table arm that I scraped for the  aluminum. I just had to bore out the center hole to the right diameter and tap the screw holes. 
Jack

The size of the wire you use has much to do with the design of the scope and also whether it is a variable power or not. The design issue has to do with the placement or location of the crosshairs in the scope, are the crosshairs at the first focal plane or the second focal plane. If at the first they will be magnified in size by the power of the scope and at the second the power of the scope does not affect their size. This is why on some scope, I have a Swarovski variable power in which the crosshairs are at the FFP so as you zoom up the crosshair enlarge by the magnification. (This particular scope is a 1 1/2 to 4X power) On this type of scope you would want to start with quite fine wire for the crosshairs. But on some Leopolds (The worlds best 2nd rate scopes) their crosshairs usually or at least on everyone I have that is variable has the crosshair at the SFP and the crosshairs stay the same size throughout the range of power zoom. On this type you would pick a little bit larger wire diameter. You get to chose. On the scope I built way back it was aprox 3 power and had the cross hairs at the FFP so they were magnified by the 3x power.