SHOOTING TOMORROW - Trouble with viewfinder masking

[Nrk]

Have spent the past few days sticking hundreds of eyes up on the walls, ceiling and floor of a warehouse and am about to go mad. Sat down with my 1014 and Nizo Pro to start framing test shots and the 4:3 aspect ratio is melting my head a bit. Thought that it would have been easy to just mask the viewfinder for a 16:9 aspect ratio but am not sure how to go about doing - either I am complete idiot or I am... a complete idiot. What do I do? Masking tape on the viewfinder didn't work and made it difficult to focus my with my eye.

I am guessing people will probably tell me to just visualize the widescreen lines but, if possible, i would like to mask the finder.

[woods01]

Yeah just imagine the crop line in the bottom 3rd of the frame. You can always adjust the crop in the telecine.