Remember too James, that scrapbook is brilliant for this stuff! Just drag your object from the scrapbook and then sample it and note away, all will be same.

How about hiding the edges at the seam line and any faces in that seam in each group in SketchUp? Then you don't need to worry about creating a mask in LO.

I do this sort of thing frequently to disguise that there are two separate components next to each other. I did that on this chair, for example.
Barrel Chair.png