The problem is mostly length. Other than that some of them are pretty well suited to being a cue case. Once upon a time someone tried to sell me bow cases which were about the right length and after I stomped one to death in my booth they thought twice about bringing them to market.
I reccomend looking around though at gun cases, bow cases, and tripod cases because often you can find something that isn't too long and will work well.
The only other issue I have found is keeping the cues secure. Most of these cases are made with egg crate foam where the piece is meant to be laid flat and compressed between the foam. This is ok for a single piece or even two pieces or any number of pieces but it's a bit of danger as you MUST have the case laying flat and secure before you open it or your cues will tumble out.
I suggest getting foam from a fabric store and using an electric bread knife to cut it so that your parts can nestle in the foam and be secure.
Or some other equally effective way to secure the parts.
John - has bought many gun cases, instrument cases, tripod cases, bow cases, and all sorts of cases over the years.