unistrut channel on wood post
If you use strut get the 1/4-20 strut nuts and they will hold the panel to the strut with no problems, make sure to lag the strut in place. I use strut regularly for mounting when the box(s) or disconnect(s) need spacing or proper support.
Make sure to debur the edges if you cut it or you can end up getting a nasty cut putting the nuts in the channel.