Dave, yeah that is something to keep an eye on with the N5c. I was using the quick push-on mounts everywhere but at the front exhaust side and using the original thumb screw there only but over time the threads started wearing and too often that mount had started to unscrew in flight. I got the idea for the Quick UK magnet mounts from another team member and use them on the front and key-chain mounts on the rear and I'm quite pleased with that setup. Spacing is critical and I too have used some tail blade washers to help.
I am interested in Gasser Mike's comment on using the N7 front mounts on the N5c, I just got a set to put on my E7 but he's got me thinking!
Good luck,
Bob