So like I said, I strongly recommend using hot glue for this rather than duct tape. Hot glue actually makes it more secure and I think it looks much cleaner.
NOTE: I did this the first time; MAKE SURE YOU DON'T GLUE IT ON BACKWARDS! It's easy to forget really.
Once you get your sense of direction, find a good spot where your monocular? fits relatively well or interlocks. DRENCH the bottom of the scope in hot glue, then stick it directly into the desired place. Once that batch hardens, try to cram some more hot glue into extra holes or concealed areas to guarantee a strong bind.