I haven't found toe covers or shoe covers that fit OVER the Keen sandal. Between the wide width and the thick tread, even XXL covers don't fit. Shoe size is mens 9.5/womens 11C.
The shoe covers I've tried are also too tight on my thick ankles and lower calf, regardless of the shoe.

I can loosen up the Keen sandal enough to fit a fuzzy thermal toe cover BETWEEN sock and sandal. This is my preferred solution for cold rides.
The toe cover cleat cutout doesn't bother me when I am wearing thick (Blaze) wool cycling socks and pedaling. Not so great when walking and uncomfortable with thinner socks.
The Keens have a rubber toe bumper that completely blocks wind to the toes. And I've decided that my heels do not need extra cold protection, they have good callouses.

In N.Texas most of my rides are above 40F and I do not ride on cold rainy days.
However, I did order some low-cut neoprene booties designed for pool swimming with fins. I think they should also fit between sock and sandal for cold weather.
I tried Performance neoprene socks but the XXLs were too tight and hard to get on, came up too high on my leg, and couldn't be worn over a sock. They were intended for rain.
I've also seen overshoes (goretex?) online that should fit over a sandal but haven't tried them yet.