Finding what Robert Smithson wore was really easy. He doesn't look like he belongs in this space at all. There are two outfits (what do you think about combining them both or one take the role of the waiters and the other the black "I'm a NY artist" look?).
Smithson with Richard Serra, 1970
Smithson and the Construction of Spiral Jetty, 1970
Smithson and his waiters (I call, "Not It" on wearing this one)
I cannot find anything (as of yet) on what Nancy Holt wore. I surmise that this is her in the views of Sun Tunnels but cannot be certain. Whatever she is wearing, she is NOT trying to be a cowgirl or cowboy for that matter.
This is the Nancy Holt we will not be emulating:
Heizer is also easy to find. Here he is with Robert Smithson in Nevada in 1968 (I am counting on finding a white cowboy hat). Here's a more recent headshot.
And James Turrell... Oh! The black cowboy ensemble! Maybe you can buy the white gear and I'll buy the black and we'll really be covering a lot of territory.
What is so odd is that I really want them to be wearing something like in Richard Prince's Untitled Cowboys photograph from 2003. Dream on.
Of course we can always dump the whole idea and go looking like this: