I take mine off when polishing the wheels and when I put them back on, I tweak the clips slightly/gently so to create what I think is a little more tension.
When I started taking them off and putting them back on regularly, on a whim, I had some Loctite spray (
CLICK) sitting on the shelf so I applied some thinking it would provide just a bit of tack to keep it secure. So far, I haven't had one come off while driving and they seem nice and secure each time a go to remove them again. I suppose the Loctite medium thread lock would do the same ...if putting this type of adhesive is doing anything at all. I'm not entirely sure.
edit: I've never tried this but I remember somebody at an RV park telling me about it years ago ...he put black friction tape on each clip.
Also, the link that Davdeb1 provided takes you to the Raney's Truck Parts home page. Here's a more specific page that He may or may not have intended us to go to ...click
HERE. Take note of the lip size of the one you need. Thanks Davdeb1.
If you desire the OEM Freightliner ones and need the exact Freightliner part number, call FCCC in Gafney and give them your VIN and they'll give you the part number for the ones you need as there are several styles to fit different wheels. I believe you can also order directly from them but as you say, it will be much more expensive.