Best shelter is a large plastic storage bin with lid tied on and a hole cut in the top at one end then the thing laid on its side. I have the porch furniture cushions in the one I made for the two porch cats that have adopted me.
I would think it would be more stable if it was left upright, or upside-down, and the hole cut in the bin rather than the lid. Doesn’t it rock when the kitties move around?