Guinea pigs grazing on grass look peaceful, and that is exactly the problem. They are prey animals with no defense beyond freezing or fleeing a short distance, so the run itself has to do all the protecting, against attacks that arrive from below, from above, and through the sides at the same time.
Mesh and Lids That Predators Cannot Defeat
Build the run from strong welded mesh, never chicken wire, which foxes bite through with ease. Welded mesh holds its shape when a fox throws its weight against it or a cat drops onto the run from a fence, and its smaller openings stop paws reaching through to hook a pig. Mesh gauge matters as well as mesh type, since a thin welded panel flexes where a heavier one holds. Check every panel for broken welds and rusted corners, because a predator tests the weakest point, not the average one.
Cover the top with a securely latched lid a cat's weight cannot shift. A loose lid, a lid held by its own weight, or a simple sliding catch a fox can nose open all fail the same way, so use bolts or proper latches at more than one point and tug the lid hard before every session. The lid also answers birds of prey, turning an open-topped run from a visible target into a covered space a hawk cannot enter.
Stop Digging Underneath and Give Them Somewhere to Hide
Fit a mesh skirt or solid base so nothing digs underneath. A skirt of mesh laid outward around the run and pinned or buried defeats a fox digging at the wall line, while a solid base under the whole run removes the digging route entirely on hard ground. Walk the perimeter before use and look for scrapes, loose pins, or a gap where the ground dips, since security fails at the one unexamined corner.
Include a sheltered hide inside the run, with an entrance your pigs can reach in one dash and enough room for all of them at once. A hide is security, not furniture: it breaks a predator's line of sight, gives the pigs somewhere to bolt when a shadow passes, and provides shade and shelter from a sudden shower. Position it away from the mesh so a reaching paw cannot find a pig pressed against the wall.
Supervise, Site Carefully, and Bring Them In at Night
Site the run where you can see it from the house, move it regularly onto fresh grass, and supervise grazing rather than treating the run as a place pigs can be left for hours. Bring guinea pigs indoors or into secure housing overnight, when fox risk peaks and nobody is watching. Inspect latches, mesh, and skirt each time the run moves, because transport loosens exactly the fittings security depends on.
Those checks often rotate around a household, and they only protect the pigs when every person performs them fully. Rotate who does the inspection, but never the inspection itself: a fresh pair of eyes notices the frayed weld or the dipping corner that a habitual checker walks past, while a shared written list keeps the standard identical whoever performs it. Replace damaged mesh panels rather than patching them with wire, because a patch is a weak point with an edge, and edges are what predators work on. KinsLe keeps that standard steady through Family Sharing, which gives each helper one shared record for the pigs, listing run inspection notes and the housing routine, so the lid is latched, the skirt is checked, and the pigs come in at night regardless of whose turn it is.