pop hole

Could a fox dig underneath the run? If not, then as long as the door faces away from prevailing winds, then you should be ok. A few people down our allotment site don't lock their birds up at night and the birds are fine.

It sounds like you have a similar set up to me, except I do like to lock the girls up at night no matter how late I get to them! Others have a rat problem, but they have old sheds adapted to chicken coops and use layers of chicken wire that rats can gnaw through. The steel mesh should prove difficult but not impossible for them to get through. Best thing is to clear food away each night as much as possible. My girls are messy and throw lots of pellets on to the floor of the run looking for the exact size they prefer  so this not always practical. Just keep your eyes open for signs of rats. Hopefully you won't be troubled by them, like me 

Not just rats a cat or mink could get in there.

don't take the risk 

better to be extra safe than sorry