This mount fully supports the edge of the Arduino Uno PCB to protect it from flexing during plugging/unplugging the upper boards.
On 1/7/15 I uploaded a slight revision. The mounting feet are a little bit stronger now.
Mount frame using #4 screws.
Mount Arduino onto frame using #2 screws.
I ran the STL file through Netfabb so hopefully the model is error-free.
There is a small tab that I locate towards the left front corner. The Replicator goes there first so that initial blob of plastic is not part of the finished part. The tab seems to help keep the part from lifting in that corner too.