Summary

Specify outer & inner diameters, height and number of facets, then slice and print.

If you're not quite sure what size washer, spacer, plug, or whatever it is you want, play around with the settings in scad. The whole scad file is only 26 lines of code.

Your exact specifications are printed as the middle object, surrounded on the left and right by +1/-1 sized. If you want a solid plug, set the inner diameter to -1 and optionally choose a bottom outer diameter larger than the top.