What I'd do is tape the lens in the exact position from the front then use a small tool to run a thin bead of silicone around the perimeter on the inside such that there won't be any shadowing.
Give the lens a good clean & polish first.
If you look at the housing you'll see it has spring-loaded fingers on the sides. Try bending them out slightly. Or it may be that the problem is that the fibreglass panel is a bit broken out around the hole, in which case you'll have to repair it / reinforce it from the back.
Fiddly but do-able.
New ones are available & not very expensive if the chrome on yours is pitted & the lens yellowed.