Everybody seems to have their medium. I know one man that loves to fix
things with bungie cords, duct tape, and silicone. (No, it's not me.)
He can attach a heavy side mirror assembly like it's bolted on using
only silicone, and it's hard to detect too.
You want the lens to be water tight so perhaps silicone would work for
you. You'd have to tape it in place while it cured. Clear epoxy would
work too. If I thought I'd ever need to remove the lens, say if the
bulb comes out the front, I would instead drill two small holes in the
plastic and attach it with screws meant for plastic. I have access to
every type of screw, but if you needed something you might rob screws
from the back of some Chinese junk you are scrapping out. Look in your
trash. We all seem to have some.