I printed the first version without the 'second stage' attachment and test fit it to the first stage, tweaked it a little and then proceeded to add the 'second stage' attachment in onshape. I printed it and it was perfect.
See also https://www.printables.com/model/535029-replacement-part-for-broken-camera-tripod and https://imgur.com/gallery/5QzL1UQ
I printed the first version without the 'second stage' attachment and test fit it to the first stage, tweaked it a little and then proceeded to add the 'second stage' attachment in onshape. I printed it and it was perfect.