A stink pit. You basically create an area where you dump all of the dead stuff, and at this time of year afterbirth, dead lambs etc. Any vermin you shoot etc. Normally you would create only a certain number of routes to it and set snares on them. But you could make it somewhere that is hidden away, so the fox feels safe, rather than in the open, but with maybe a high seat put up for you to ambush them? Then try early mornings and evenings.
Obviously baiting them requires a removal of easy alternatives, so if he lambs outside and there is afterbirth lying all over the fields.. . . .that mi