I'm am quite surprised that I managed it too since I know nothing about optical design. I had quite a few lens elements at my disposal from various lenses which have evaded repair over the years.
I found that quite a large positive lens at the front and negative lens about 3/8" behind it coupled with another positive lens at the eye position spaced about an inch from the negative lens worked relly well.
This is one of those things that needs three hands. Holding the lenses and working out the relationship between them.
I found that the 3/8" between the front and middle elements was not very critical but the spacing of the rear element in relationship to the middle element was crucial to get the focus right.
The lens does introduce a bit of distortion. In order to make the rectangular mask appear straight in the viewfinder, I had to cut it to a slight pincushion shape. I know that the finder may not accurately show exactly what is being projected to the film but I am confident that the angle of view is the same so as long as I line it up based on comparing the viewfinder image with an image on a temporarily fitted groundglass, I should have a good idea of what I am pointing it at.
Anyway, it's such a wide view that I am going to have more of a problem keeping things out of the image than in.
Changing the subject, I was also planning on fitting a spirit level to the top. A few days ago, I found, in my shed, an old Rabone wooden spirit level. These have a brass plate with the manufacturers name engraved on it fitted to the top with the glass tube fitted below. This one has rotted away a bit so is of no use any more so I am thinking about routing out the camera's top (which will be oak) to take this plate and fit the bubble tube below it.