I have designed a 3D-printable focusable FocasScope for the Kalart Rangefinder and posted it at thingiverse.

As designed it holds
- 1.38cm diameter convex lens with focal length 48mm <= f_1 <= 72mm
- 1.67 cm diameter concave lens with f_2 = 2 f_1 /3
- the lenses should be placed f_1-f_2 apart which can be adjusted by screwing the pieces relative to each other. The distance between the two lenses can be adjusted between 16-24mm

I obtained my lenses from American Science and Surplus

for other lens diameters and for focal lengths requiring greater distance between lenses, the model must be adjusted.

It fits and works well on my 1945 Miniature Speed Graphic.