Once a FrameUnit is mounted to the MiddleFrame, you will notice the structure will tip over slightly. While you are building the model you can prevent tipping-over with this little support.

Print a couple of this model at once and use one of these supports on each already mounted FrameUnit.

Its a friction fit, so press lightly onto the UnitFrame.
Scale the length to your liking/stability needs.

Remove them once you mount the FrontCover pieces.