You can also try mounting the light on your left seat stay, assuming you don't have a rack in the way. This puts the light lower to the ground, which may make it harder to see, but also angles it up towards the drivers' eyes, so it probably evens out.

The problem I have with clipping most lights to most wedge bags is that the light often ends up sagging and pointing towards the ground.

I don't know of any brackets explicitly made for this purpose, though Topeak makes a seatpost bracket designed to mount a water bottle or one of its wedge packs. You might be able to modify this to clear your wedge pack and hold the light. Here's a picture of the Topeak gadget:

Topeak S.P.P. Fixer