This is the reason I've gone away from saddle bags. There's a limited amount of precious space on the seatpost/seatstay that I'd like to use for lights and a rear view camera. I've done away with the bag altogether. Instead, I will be using a plastic canister the size of a water bottle that goes into the seat tube cage.
In the past I've tried mounting a light onto the saddle bag and had the same problem as you. The backpack is no good either.
I also like to mount multiple lights on the back, one of which is always on blinking. I found that my SKS Longboard fenders are a good spot for mounting a light if I have my 32 mm tires on. With 35 mm studs, this can't be done. I realize that's not an option for you with the flimsy MK3's, which are really, really ugly, BTW
I have the older MK2's. Much more subtle looking.
