Being only a few days from August this might, at first, seem to be too late a reply to the question posed here, but it might be of some benefit in the future.
I'm in E Idaho, and this past March our local chapter of DU put out over 40 Wood Duck nest boxes, but our big problem was not so much whether we were too late, but too 'early' in a manner of speaking.
We got our boxes installed in plenty of time for the Woodies, but also for the Starlings, who simply filled over half those we installed before the Woodies could set up housekeeping.
Now I'm of a mind to screen over the holes on those, and future boxes, until the middle to end of April, or until most Starlings are committed to nest elsewhere, and then open them up for the Woodies.
I don't know if this may be too late, but we are in the Northern/high desert part of the Mountain west, and I'm hoping for a bit later nest season for our local birds.
Can't think of what else to do about the Starlings. Guarding every box, or even a weekly check on them is a bit out of the question because of their distribution over a considerable area.