John Lewis in London used to have special darkroom blackout material. That's what I got when I was there. I cut it to size, put velcro all around the cloth and the window frame and simply stuck it up for printing sessions. However, light is quite obstinate, it still goes through in between the velcro. A heavy curtain on top dealt with that.
You live in a wonderful part of the world. I was fascinated with Glen Coe. We hiked over the saddle point between Buachaille Etive Mòr and Buachaille Etive Beag.