Curious Happy Jack what did you make the rain cap out of? Before fitting my through the roof mount I got a 50mm pvc end cap and used the heat gun. Placed hose clamp around the end cap and heated it while over the roof mount, once clamp got tight from heating the cap. let cool and it is the right size. Painted cap , hose clamp and roof mount to same colour as roof and very few notice it from outside. When need to fit light loosen clamp and tap light from under neath and cap is removed. it was all I could think of at the time.
PS the police tend to pull me over in the city when the spotty is fitted to the old 60 crusier and this makes my life easier