Better fans come with a set of special screws that cut into the plastic. There are also plastic expanding pin mounts (work sort of like pop rivets) that have come with some of the cases I've built out, but I don't know where to get them separately. Startech.com and svc.com et al. have the special fan screws - most of the SVC brand fans come with a set of screws and a chrome guard.
. It's possible that you could use those expanding plastic mounts that are used to fasten stuff to wallboard and instead of using a screw to cause the expansion, use a short piece of wood or plastic doweling. They probably come in a size that would fit in the fan mount holes.
.bh.