It goes along that same thing where people leave styrofoam coolers out for outside kitties in the winter. I tried that. Once their nails hit that styrofoam, it breaks off into little bits that gets into their fur and then of course their natural instinct is to clean themselves and thus they're ingesting those little pellets and can't imagine it's healthy. These will still allow for insulation.
I took it a step further and measured out an outside crate on all sides - same could be done with any container. I created filled pillows matching the size of each side with an additional couple of rows for the "seams" and then single crocheted each piece together. For the front, I did do a plain flap that would just cover if needed and they're able to use it as their own lightweight door and it can be tucked under the top to keep it out of the way as well. Just remember to only connect the door on the top leaving the edge so they can get in and out. It's more functional than beautiful! (Make sure to place it on a higher surface to keep them safe from predators. Some generally like a double-exit method (I have one that doesn't) but this doesn't lend itself to that.)
These can also be used as mats. Have also seen reusable shopping bags, rain hats, coasters (though it wouldn't protect a surface well), and many other things. Just adjust your hook/needles accordingly. And maybe even the size you create your plarn. You may not want as thick of a cut for some projects as you would the mats.