I obtained and sealed up (using mastic) some 12v PC case neons and had them on my front bumper for a while. They were only stuck on...as a result they rattled loose and broke off...
Id ideally recommend bending a mounting plate out of some thin steel (0.85-1.1mm), and then screwing that into place using some self-tappers.
Oh...a bit of ingenuity never goes amiss...I cant tell you what the neons you buy would be like, unless you buy the exact same product, even then, design revisions may affect what I can tell you.
You need to get the things then solve the problem. Dont try solving a problem if you dont know what the problem is!!