How to make an insulated cat shelter for outside cats?

You could take a tot and a smaller one and put towels between and news papers that will work fine

think the spray foam insulation would be great - but at least 3" or more; I'd use TWO totes -- one smaller than the other and spray the insulation between the two totes

- styrofoam coolers will deteriorate and are not as 'cleanable', cats may dig with their feet, etc.

I'd use a good quality feed hay - not straw -- on the inside, and make the unit small enough so that the cat can heat it with it's own body heat; You can change out the hay as necessary but it won't absorb moisture from the atmosphere or a cat's wet body from the rain like wheat straw does. Towels/rugs get nasty, harbor fleas, etc. Make the entry hole to the unit on one side of the lid ... not right in the middle -- that way wind will not have a straight shot into the box, and the cat can have more 'room' to the side ...

Be sure to place these shelters on a rack/stand up off the ground; from a couple of feet to several feet -- cats like being 'up high' with a good visual vantage point of their surroundings.

Check out these on pintrest

https://www.pinterest.com/search/pins/?rs=ac&len=2&q=cat%20shelter%20outdoor&eq=cat%20shelter&etslf=9448&term_meta[]=cat%7Cautocomplete%7Cundefined&term_meta[]=shelter%7Cautocomplete%7Cundefined&term_meta[]=outdoor%7Cautocomplete%7Cundefined