Fit a chainguard when there is a front derailleur
There are a few plastic chainguards for front derailleurs.

I never found a full chaincase but managed to fit this de Woerd Wave chainguard to a mountainbike used for commuting (3&times;7 gears I think).  I failed to get the same model working on my 3&times;8 hybrid, though it had the same chainrings as the MTB. I would go on, but I couldn't avoid it rubbing in some gears because I couldn't get the bottom bracket clamp to sit straight, and never got round to modifying it enough to make it work to my satisfaction.

Holland bike shop has a wider range of de Woerd parts; some are specified for double or triple chainrings.  I'm sure there are other makes, but these are the ones I've come across

A double chainring would be easier to work with than a triple, but you may have to modify the chainguard a little - easy if it's the fairly soft plastic part, but harder if it's the mounting brackets (which have to be secure to be safe).