My children are used to sleeping in nearly complete darkness (just a night light switched on), so whenever we travel to the Nordic countries or places without proper shutters, or when we stay with friends and sleep in rooms normally devoted to other uses, our nights tend to be shorter than usual.
I have tried many DIY methods to get less light into the room, some more permanent than others depending on the length of our trip. I have hung towels, blankets, double layers of blue fabric using self-adhesive velcro strips, etc.,  and was rarely satisfied with the result.
During our last trip to London we used baa-baa blind on the window, and it worked really effectively with very little effort! 
These blinds consist of a black-out material measuring 200 x 130 cm with eyelets on three sides where you can fit suckers that will attach to the window. There is also a hidden ribbon that allows you to shorten the blind if the window is too narrow. Because they attach directly to the glass they have a very close fit which blocks out most of the morning light.
They come with a nice storage pouch to hold the blind and the suckers — ideal for travel now that the days are getting longer!

-Michela