a lot of bike lights will have a mounting clip like this - i have this one and it fits reasonably securely on my GR2 and on the bullet.

you may also consider getting a few reflective bands (like this, but i should emphasize you could fashion one yourself), to give yourself a bit more visibility (especially on the sides). i found that when i was hunched over on my road bike the rear light would face more up than back unless i set more upright, which isn't how road bikes typically are designed (usually more hybrid and mountain bikes)