There are some preloved metal dog kennel panels on ebay. Anything that will keep dogs in will also keep foxes out, as long as it has a decent roof and solid or mesh floor. You could put smaller grid weldmesh panels on the inside along the bottom meter to make it safer for the rabbits (fasten with metal cable ties) so they don't get stuck in the bigger mesh or bars. Put the whole thing on some cheap / used concrete slabs (eg Freecycle or local FB groups). The roof can be weldmesh or corrugated or twinwall plastic panels. You may be able to find a second hand wendy house or similar to put inside it as their base.