Double the fleece?
Lots of deep straw/hay bedding....gets it fluffed around them and it helps hold heat, breaks wind, etc. Be sure to pile some around the bottom edges of the stall if there is any opening where air comes in, especially at the stall door this often happens....remember they are laying a lot.
If you have a really "open" stall, hang tarps outside of it to keep the body heat from the mom & foal contained. Have an overhead light socket? Use a 100 watt bulb, it will help heat the stall air but only good if you can minimize air movement where it is........do not lower to where it can touch any straw, horses, etc.......obviously, close all barn doors, etc.
It does take a couple of days for a foals system to be able to regulate it's own temps well.