Curtains in hotel rooms are often not thick enough to block light from outside. They also don't cover the edges of the window well. Last year in Norway, it was difficult to sleep with the sun shining into the room at 2 am in the morning.
Rather than making your hotel room dark, you could use an sleep shade (photo from Amazon, but there are many types available). Also called a sleep mask.
It's worn over the eyes, in case that isn't obvious. Another photo from here:
With limited ressources in a Hotel I can only think of a couple things.
- Spare blankets
- Used towels
- other fabrics
If you don't have a curtain rail to hang it over you can try to put it in the window. Open the window slightly and tuck the fabric into the top egde of the window. Don't squeeze to much fabric in there - windows are already leaking your warm air out.