I have a few red window cameras that don't have covers, indeed I have one, and Ensign 845, that doesn't even have any red widow at all, just the frame number with nothing, and after several using them I have yet to have a fogged negative, indeed when using the cameras with red windows that actually have a cover I very rarely use them,so if the cover is not there I wouldn't worry about it, never yet had a fogged film, even when using 400 asa film
Plain, old electrical tape is unlikely to hurt leatherette, provided you don’t leave it on there for a long time in hot weather. Even then it might get a bit gooey but it won’t hurt anything.
For a slightly higher budget, black gaffer’s tape works well for this kind of thing but be careful; not all gaff tape is created equal and the cheap stuff can make a really sticky mess.
I usually don't worry about it.
If I did worry about it, I would use low adhesion painter's tape when I had the camera out, and a camera bag when I wasn't using the camera.
I use electrical tape and it has yet to ruin the leatherette on the camera backs. I do this for Agfa, Voigtlander, Zeiss Ikon and Kodak.
I also use it when the red window is missing. I also use the covers when provided by the original manufacturer as I have always believed that they knew what they were doing when they built their cameras.
Obviously I must use my cameras differently, or use different film, but if I leave the window open (the one missing the red insert window) I will most certainly get fogged film. Mine are 6x9 cameras, perhaps it is different when you are using a center window like that used for 6x6 format.
Actually, the only reason the windows were red was because when roll film cameras that use backing paper first came out the film available wasn't sensitive to red light.
With panchromatic film, dark green would actually make more sense, because our eyes have enhanced green sensitivity.
6x6 and 6x9 plus my 6x12....no issues with any of them...with pan and colour film. My Zeiss-Ikon has a slide cover and I do use it, which has ultimately lead to the red window being scratched by the sliding cover!
OP are you actually experiencing issues due to the red window?
Depending on the model of camera: Large fridge magnets can apparently work well... Assuming the back cover is sheet steel and the camera covering isn't too thick or there are any excessively large bumps. Simple flat magnets hold with surprisingly reliable force.
Beyond that, I think the most graceful option is to make some manner of camera cover/ever-ready case with a quick open rear flap, even if it is just for the back.
The two cameras I have that have backing paper windows all have little slides built in, but one is getting kind of stiff, so I may switch it out to a full back cover if I keep using it.