Adrian,

I often find it helpful to use an open-frame finder; no optical parts to intrude in terms of magnifications (positive or negative), no chance for fingermark, perfect brightness etc. After all it is an aiming device, which frames a slice of reality before you within the four edges of a rectangle.

Leica, among others, produced frame finders, their designs can be used as references for constructing your own; one for a single focal length would be a simple job I am sure.

Good luck!