Review on Enhanced Protection for your Data: Tripp Lite Datashield Serial In-Line Surge Protector, DB9 by John Brouillard

Depending on what you're mounting them to, you may need to remove and swap out the screws that attach them to the unit, but this is very easy (and mentioned in the manual). Installation was pretty darn easy. Ideally, I'll never know if this actually works, but I've learned the hard way that lightning surges can get down unprotected RS-232 lines, and I have an expensive audio preamp that has suffered the consequences, so here's it lesson learned.