Mounted internal to the window frame, not external, works better for me. Internal can ride tighter to the window, so light can't go out the edges. With external frame mounting, you need much wider shades.

If the fabric itself isn't blocking light... You need better material. I have only ever had problem with light leakage in the edges, not in the fabric material.

I believe 'blackout thermal shades' is what to look for.