Kestrel numbers have been sliding for decades across much of their range, largely from the loss of natural nesting cavities — old woodpecker holes in dead trees, which get cleared out of managed farmland and suburbs faster than the birds can find replacements. A well-placed nest box is one of the few interventions an individual landowner can make that reliably helps.
Kestrels want open hunting ground nearby — a field, pasture, or large mowed lawn — with the box mounted 10 to 30 feet up on a pole or dead tree, facing away from prevailing wind and, ideally, with a clear flight path in front of the entrance hole rather than a screen of branches.
Boxes should go up well before the breeding season starts — kestrels begin scouting cavities as early as late winter in much of their range, and a box that's already weathered in and familiar gets checked out sooner than one installed at the last minute.
Once a pair takes up residence, the best policy is distance — check occasionally from afar, but resist the urge to open the box during nesting; kestrels will readily abandon a clutch if disturbed too often early on.