Biohazard

Okay...so the right way to do the lenses is......

If they are still there, you should have plastic posts onto which the metal tabs secure. With the plastic posts sticking up through the holes, you then use a soldering iron to melt the tops of the posts, effectively mushrooming them down over the metal tabs.

Failing this, probably due to not having enough material left at the top of the posts, you will just need to resort to the adhesive of your choosing or you can try to source some plastic to use to fuse with the nubs using that soldering iron.

I used a combination of these. Where the posts were tall enough, I melted them. Others got glued. All got a dab of epoxy as a bit of extra insurance.

No matter how you proceed, take care about pushing on the lenses while reinstalling the gauge faceplate.