First of all great progress, especially considering the very short time frame! Your back is already widening up nicely.
Shrugs, dumbbell or barbell, will work your upper traps. Try with no weight to begin with so you know what the full range of motion looks/feels like, then add weight. Try not to roll your shoulders back and forwards when doing this, just an up and down motion is all that's needed. A short 1 or 2 second pause at the top of the rep is a nice addition.
That will work your upper traps anyway. If you want to target your lower traps, and please correct me if I'm wrong anyone as I'm not 100% certain, scapular contractions is what you want to be doing. Basically the same movement, but on a different plane of motion. With your back parallel to the floor, shrug your shoulders upwards as if you're trying to pinch your shoulder blades together.
Is this the only lower trap exercise that can be done with free weights out of interest? It seems to be something that a lot of people neglect.