Those parts are keyed to go only one way. Look for the half circle on one side and orient it into the corresponding hole. It goes into the upper side. It appears from your picture that it is in the lower side.
Of course it was the obvious. Didn't notice it was to be mounted in the top. Now feel human. @CambriaArea51 nice build and thanks for the photos. This is the 2 bay ACF hopper I have some 2750's and they are a little different.