Brian Manning made a nice one using a 3" f/15 refractor and my unit which I'm working on now is similar in design. It uses 70mm Celestron refractor.
http://adsabs.harvard.edu/full/1982JBAA...92..112M

There was one build by a high school student written up in Sky and Telescope in the 70's made from two 50mm bino-objectives and inexpensive transmission grating but you really don't want to go smaller then around a 45" fl for the spectroscope section or the width the spectral line is narrower and that requires a narrow slit openning.
For a spectroheliograph used with a webcam, the spectroscope section can be very small. An achromat with about 7" focal length and a 2400 lpm grating would work very well and teamed with a 60mm to 80mm refractor would make for super imaging system.

- Dave