Not aiming for style points here, but instead for ruggedness. Steel is more rugged than resin.

I bought the $8 cigar case as an experiment to see if a pen would fit. It did. Nicely.

A stainless steel cigar case fits my TWSBI Echo plastic fountain pen nicely for stuffing it in a pack or my pocket.

The picture has a blank 3x5 card for size reference.

So if you’re not too concerned about of what people think while you are out and about, this is a nice solution.