I need to sandwich some vertical 4 x 4 pieces of pressure treated
Doug Fir with two 2 x 6 horizontals at the top to support an
outdoor patio cover.

I could just use some 1/2" allthread through all three pieces
of lumber, fastened with washers and nuts, but I dislike the
look of the protruding hardware.

I thought of using a tee nut on one side, but the only supplier
of those parts in 1/2-13 apparently cannot ship them.

Now I'm thinking of removing the hex from the outside of some
coupling nuts so that I can use them to join the ends of a
couple of button head recess hex cap screws with washers.

Is there a better way to do this?

Thanks!

--Winston