Guinea pigs are prey animals with no defence but the barrier you build for them, and an outdoor run is only as strong as its weakest corner. Foxes dig and chew, cats reach through gaps and work at catches, dogs test panels with their weight, and birds of prey watch from above. Predator-proofing is therefore not one feature but a system — mesh, floor, roof, and latches all holding at once.
Mesh first: welded wire, small gaps
The commonest failure is the mesh itself. Woven chicken wire is made to keep poultry in, not to keep predators out: a fox can bite through it, a dog can tear it, and its wide hexagons let a cat's paw reach a guinea pig sheltering against the side. Choose welded wire mesh instead, in which every join is fused, with gaps small enough that a paw cannot pass and a guinea pig's head cannot get stuck. Cover every side, including any low panels you step over, and fix the mesh to the frame firmly at close intervals so it cannot be peeled back from a corner.
Diggers: aprons, skirts, and solid floors
More runs are entered from below than through the sides. A fox or dog that finds the base will dig at the edge, so deny them the start: pin a skirt of the same welded mesh flat on the ground all round the run, extending outward and pegged or weighted down, so digging at the wall lands the animal on mesh it cannot get through. Alternatively, stand the run on a solid base of paving or a fitted mesh floor. If the run moves around the lawn for fresh grazing, the apron moves with it; a run that is safe in one position must be re-secured in every position.
Roofs and latches that paws cannot work
An open-topped run invites cats and birds of prey, and a guinea pig panicked by a shadow has nowhere to hide. Roof the run fully with the same welded mesh, or with a solid cover over at least part of it, and put a sturdy shelter box inside so the pigs can get out of sight fast. Then look at every opening you use: twist catches, hook-and-eye fastenings, and loose lids can all be lifted, slid, or knocked by a climbing cat or a determined fox. Fit proper bolts on doors and lids, and check that hinges cannot be sprung by leaning weight.
Siting, shelter, and the weekly once-over
Where the run sits does half the work. Place it in view of the house, away from fences and bins that give a predator a hidden approach or a launch pad onto the roof, and out of full-day sun so the pigs are not forced to shelter against one exposed side. Bring guinea pigs in rather than leaving them out overnight, when foxes are boldest and you are least likely to hear trouble. Walk the run weekly with your hands, not just your eyes: tug each panel, test each bolt, look for chewed timber, fresh digging, and mesh lifting at the corners, and fix small failures before something else finds them.
Not every keeper owns the tools for cutting and fixing welded panels neatly, and a run built badly is worse than a job handed to someone equipped for it — the Pet Services Directory in KinsLe connects you with trusted local pet services, so the mesh, apron, and bolts on your guinea pigs' run can be fitted once and fitted properly.