One of the entrances to my hog feeding box is only 10 x 11 cm and even quite large looking hogs seem to be able to get in and out, even though they have to turn and climb over a brick as soon as they get in (designed to deter cats from the food). Hogs are surprisingly flexible and can squeeze quite flat underneath things as well, if they are able to extend sideways.
The only problem is that young foxes can squeeze through quite small holes as well. I put a few links in here recently: https://www.hedgehogstreet.org/forums/topic/making-dog-proof-hedgehog-holes/ You might get some ideas from those.
But you could try putting, say, a breeze block (or few) in front of the hole so that the hog would have to turn immediately left or right which might make it harder for a fox. It might be a case of trial and error to see how close to the hole you could put them.
If you do get hogs, you will need to make sure neither your border terrier nor the hogs come to any harm. I’ll put a link in the next post (the forum doesn’t like too many links in one post!).