Is this the correct way to install a damper?

I just had a large HVAC job done and part of it included installing dampers on two vents that get significantly more airflow than other vents, making those rooms very uncomfortable. The contractor apparently installed the damper on the wrong vent and is now ducking my calls since I guess coming out to do two little dampers isn't worth their time.

Instead of getting all up in arms about it, I'd just like to do it myself and get on with it. I looked at the ducts supplying the vents and this is what I see for both of them:

http://imgur.com/a/7VVIj

There is a collar coming off of the main trunk, and then these thin, plastic ducts attached to them and covered in insulation.

My plan is to drill a hole in the metal collar attached to the trunk line, and then take a razor and cut off the plastic duct (it appears to be glued onto that metal collar). From there I can just insert one of these:

http://www.homedepot.com/p/6-in-Damper-D-6/100396929

Then I will take some tape and reattach the plastic duct back to it.

Is that the general premise of this?

thanks

submitted by /u/kubyx
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