I think the thing to realize with DIY lenses isn't that one is trying to make a good lens but an interestingly bad lens.

A plano-convex achromat, flat side to the subject with a stop a 1/3 of a focal length ahead of the lens should, I think, produce a good first pass at a soft focus lens. And since everyone likes to fiddle and diddle it might be interesting to make the stop position adjustable (or it might not - that's the fun of DIY).