Poor people's guide to survival without sleeping bags:
Unpack and lay all your remaining clothes as a secondary patched-up blanket below you, to add to the one blanket ur carrying that you'll put on top. Use one clothe to cover ur ears well.
☝🏼 this saved my life on a few occasions and during cycle yatra.
On one bus journey i realized the chaddar was too thin so the clothes went both above and below.
Also, newspapers tucked in can help as insulation.
Note: this was applicable to me in rajasthan's winter (which is in india). If you're at an even colder place then don't take it literally!