A couple of years ago, I either lost the font. or someone stole it, from the oil taillamp on our '21 Touring.

I have since found a replacement, but have not painted and installed it. Anybody got a tip on how to make sure the font is tight and secure, other than what would be obvious? I realize most any adult would be able to twist one off, but I'd like to have them tight and as secure as possible so they won't fall off.
I hate to take them off and haul them around when on the road, especially if they have oil in them.