Aesthetically I agree that the light would look better either on the top or the back of the rear fender.
I have similar fenders (Nirve Streetsweepers) on my cruiser and I had to drill some holes in some unconventional places to mount them to my bike.
I'd recommend you use some tape to put the light exactly where you want it while everything is still on the bike, then mark the holes using a paint pen or a crayon.
Take the fender off the bike to drill the holes obviously...What I did when drilling mine was take a piece of scrap 2x4 and stick it under the hole I'm drilling. This way there's a lot of contact between the fender and the wood and when you put pressure on the drill you won't bend the metal surrounding the hole you are drilling.
Use the high speed setting on your drill.
To mount the lights I'd recommend a bolt with a pretty thin head on it (you could always file it down too). put the bolt heads on the inside of the fender so the bolts are sticking through to the outside of the fender and put the light on with some nuts. Then you can trim any excess thread you don't need off the bolt with a hacksaw or a cutting wheel.