All my marker lamp lenses (except a mismatch a PO installed) are loose. I already lost one. Has anyone found something to keep them on securely that isn't so permanent that you have to break them to get them off?

I have a bunch of exterior LEDs coming; I was thinking maybe with their longer lifespan I could use a tiny bead of lap sealant...

I had the same problem, and like you I have replaced with LEDs. I used sealant to keep water out and lens on. So far much better.

What sealant did you use Steve?

I used a VERY small amount of clear RTV, just at the corner of the base where the edge of the lens seats. Hopefully they are still removable. I haven't done them all yet, just the ones that were loose, leaking and in danger of loss.