Hi @ChrisCph,
I share @dgalland 's opinion. Extension tubes and other lens-camera coupling elements are easily and cheaply found in online stores.
On my device I am using a Rodenstock Rodagon 50mm f 1:2.8 lens. Optically it is very similar to the Componon S.
On my device, the distance between the back of the lens (with the tube fully screwed in) and the front of the HQ camera C-mount (without the 5mm ring) is 80mm.
The distance between the Super8 film and the back of the lens: 109mm.
I would have liked to get the lens closer to the film, but with my projector it is physically impossible.
Here you can see my optical system:
And these are the lens-camera coupling elements that I have used:
Helical focus ring 17 - 31 mm M42-M42
To adjust the entire optical system, @jankaiser had the excellent idea of photographing graph paper
I wish you success in building your film scanner.
Best regards