The most pressing points are good ground insulation(mat to lie down on) wind protection and then dry clothes. We lose 80% of body heat through the ground. Evaporation of sweat or humid clothes cool at an extremely fast rate too. The amount of heat transfer depends on the evaporation rate, however for each kilogram of water vaporized 2,257 kJ of energy are transferred.
- Insulation mat is self explanatory, get the best you find
- Wind protection is achieved through two layers/steps: good wind clothes(jacket, coat) and good positioning of your camp using morphology of the terrain.
- bring spare clothes
Remember to bring a good hat, that covers your ears and cheeks. Always makes a big difference.
You might not be comfortable like with a sleeping bag and tent, but you'll get through the night.
When you expect rain things get more complicated. You have to stay dry at all cost, else you lost the battle against the cold. I never used a tent, just a rectangle piece of sturdy fabric, about 2 meters by 1 meter. I wax it regularly. Construction possibilities are many, depending on conditions, especially wind. Standard position is this:
You don't need metal poles, just use some sticks. As a habit from military I keep my tent as low as possible when I am only confronted with wind, so I can close the sides with some equipment, rocks, or vegetation. Imagine the previous diagram with the metal poles/wood sticks shortened and moved to the right 1/4 of the fabric length. When it rains hard, steeper tent sides will make it more efficient in throwing off water. You might consider bringing an aluminum field shovel to dig a small 'canal' around your Bivouac. If it rains hard without this canal bringing the water away, you will find yourself laying in a swamp quickly.