I don't know if maybe we're all overthinking this thing. We just put this up last week on cedar shake shingle and I cut a 1/2" thick piece of MDO about 6 inches smaller than the
sign and mounted the PanelClip Z channels to it and the back of the
sign in the shop. We first mounted the MDO panel on the building level and straight, spanning several rows of shingles, then hung our
sign, secured it with one set screw hidden in the crotch of the letter A in Congregational and called it done.
Doing it this way, does give you a little standoff, but that seems like a small sacrifice to know that it is evenly perpendicular to the wall.
Some might question the use of the MDO instead of aluminum tubing, but frankly, a good primed and painted piece of outdoor plywood will certainly last as long as the
sign does and the ease with which it went up can't be denied.