Great news! Before I ever purchased a foal blanket I used kids sweatshirts, etc. I know people have used dog blankets, or layered all different types of kid's attire.
You will want to put a deep bed of straw down -- banking it up on the sides to keep baby away from drafts. Depending on what your situation is regarding electricity, I've hung a heat lamp -- keeping it safely away from the dry straw -- and had that on overnight. Have lots of absorbent towels ready to help baby dry off initially. Don't worry -- last year we had a baby born in Alaska during cold weather, and with good attention and the 'human' mommy watching for signs of shivering, etc., this little one made it just fine. I think she even used a blow dryer to help baby dry off initially.
The stall should be roomy enough for momma to lay down comfortably to give birth, leaving enough room for you to assist should she need it. So, make it so you can comfortably be in there with her too.
This is exciting news and I'm so happy for you. I just KNEW she wasn't quite ready -- but with all the good pictures you've provided, we knew she was making some nice changes!