Hi Marra,
The nice thing about these rear lights, is that they also work as breaking lights. So if I brake they shine twice as bright as the normal rear light.
I bought a plastic cutting board (second hand) to fit the lights in. I made two layers and mounted the lamps on the top layer. This second layer below this one, creates that little extra room you need for the lampholders behind the plastic. I first tried it with wood, but thats not weather proof.
greetz
Frank