In order to keep the PCB in place inside the HP28S, I designed a simple "sled" like object to which the PCB attaches. The idea is to provide a rounded base that will keep the PCB centered across the cylinder-like shaped cavity in the calculator where the single N cell and the HP28S's negative terminal normally reside.
I love OpenSCAD, especially for things like this that are pretty easy to describe procedurally.
I made the holder shorter in length than the PCB to allow for the PTH solder blobs under the USB connector, and to allow the negative "terminal" of my PCB to engage with the HP28S negative contact spring. I like to include a "model" of the PCB in OpenSCAD, also - I suspect that there's some clever way to take 3D render info from KiCAD into OpenSCAD somehow, but my mechanical CAD foo just isn't that mighty. So, I just include a cube of the appropriate size, translate it to the correct spot in the assembly, and color it green:
The idea is that the holder is merely meant to keep the PCB into roughly the correct position so it stays in contact with the HP28S negative battery spring terminal.