it goes against what i said about a zip, but i’ve had some caratti ones before which were alright*. they have some “deep winter” ones:
planet x also do cheaper ones.
pair with woolie boolies and you might be okay.
*the problem i found with overshoes is they disintegrate if you walk around with them on (which i did, commuting and doing very long rides). i have so many pairs is various states of disrepair. because i have fat calves / ankles i often find them really difficult to actually get on and do up around the bottom of my leg, so in the end i just gave up and got a pair of winter boots: diadora “polarex plus” off ebay for £34. i am choosing to ignore the fact that the sole has come away from the bottom of the shoe (easily fixable with some epoxy, unlike overshoes which just have to be chucked when you get enough holes in them), because other than that, they have kept me warm (50km @ -2 degrees last night) and dry in downpours.