So I would recommend just providing lots of straw in the boxes for them to snuggle into and maybe some old blankets if you have some. Keep the boxes small. Cover the entrance hole with a piece of cloth to help keep out the wind. Put the entrance facing south and in an area that get lots of sun during the winter. For the summer put them in the shade. If possible put them in an area protected from wind and rain.

Styrofoam (at least an inch thick) will work as an insulation but you will need to put it on the inside of the boxes for maximum efficiency and effectiveness.

By the way…thanks for helping to protect the homeless.