All you need is the magic lantern markers. Then when you have your 1.78:1 image in your editing programme, divide 1920 (1) by 2.35 and set the value the video height to get the correct crop.

What is the problem ?

Please tell me this is a joke?

For your next project, if you want to be real cheap and not spend money on a monitor or anamorphic or anything, you can put your camera on a tripod and set it in front of a 16:9 TV. Line up the corners if the cameras's viewscreen, with the corners of the TV. Line them up exactly, so what you see through camera is the TV screen only, with no border of the screen showing at all.
Then pop in a DVD of a 2.35:1 movie and let it play. Then with the camera, lined up with the TV screen, take a photo of the movie. You will know have a photo with the black bars from the DVD movie on your cameras view screen. Using the black bars on the photo as a guide, put black tape over top of them, and line it up exactly when you do. So you should now have a 2.35:1 aspect ratio screen on your camera with the black tape, or at least really close, for cropping in post.

Stick with what you've got. If you must, crop it slightly to 1:85.1. I like 2:35.1 as much as the next guy, but it isn't the be all end all. Tarkovsky, Kubrick, Bergmanthey all made due without it (besides two films I believe).

You shouldn't change your ratio to something other than what you had in mind when you shot it.
A proper movie has to be planned from the beginning. If not, why even bother ?

You can also shoot video with an anamorphic adapter, but his quest is not for the display ratio, he wants the black bars, so just masking the finder in production and masking the image in post is a fairly easy and quick way to do it.
And it is a lot cheaper than buying 130,000 feet of film, paying processing, edit, post, and final match which is still less than you will spend on Craft Services for a real production.
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Or you could just shoot on film. If you want that 2.35:1 aspect ratio without chopping up the image, just shoot 35mm with an anamorphic lens.
I doubt you have the means to shoot your film in Ultra Panavision 70, let alone IMAX. So anamorphic will give you the look you want.
C. Martin Croker 1962-2016 RIP

I believe Sony Vegas editing software has "event pan/crop", which allows you to shift the image down to preserve headroom between the black bars, but then you'll still lose footroom. Yes, it's best to crop-protect for the black bars during the shoot.

The aspect ratio should be determined before you record your first shot. To avoid the issue you describe.
To guide composition, all the monitors on set (including the one the camera operator is looking at) in some way display where the image is going to be cropped. Most professional monitors will have selectable display settings to toggle through for the most common aspect ratios, and a rectangle is superimposed over the image. My monitor has red lines, but others draw dark translucent bars at the top and bottom of the image.
If you don't have access to one of these monitors, you can just put tape. I've seen people use electrical tape, but I liked using clear tape. That way you can still see the line, but you can also see what you are leaving out of the frame.
A quick and dirty method is to find an image online that represents the aspect ratio you wish to use. Print it out, and put it front of the camera. Zoom in to just where the left and right edges fill the frame. Put tape at the top and bottom of the monitor that roughly matches the position of the black lines. It's not perfect, but it'll be close.
If it wasn't for disappointment, I wouldn't have any appointments.

Do people frame their shots with the black bars in mind or something? How do I get around this issue?
Yes, each shot should be composed with the aspect ratio in mind, so that the film doesn't end up with the issue that you're now struggling with. I would suggest just sticking with your original aspect ratio for this project (I assume 16:9?). It may not look as cinematic, but I would argue it's better than cutting off your talent's head with black bars.

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