I love the hefty glass build plate that BCN3D produces for the Sigma range of printers. Not only do I use it on my own Sigma R17, but I've adapted other printers to use the same glass bed. The best thing about that consistency is that I can maintain different surfaces on the plates, and just swap them around depending on what I'm printing on which printer.
However! If there's one failing in the design, it's that the magnets inevitably become detached. And, they really do need to be precisely located, or problems will ensue! I thought I'd somehow damaged the gantry on my Sigma, but it turned out to be a poorly-positioned magnet I'd reattached to the glass.
I'm going to get to the point eventually, I promise!
And, you've already worked out what this object is, because there are photos.
But, to make a short story long: BCN3D open source all the Sigma details, including their assembly process and the jigs they use for doing so. With some google translate help, I located the technical drawings for the bed jig, took the appropriate dimensions, and built this tool!
Hopefully, it's obvious how it works - just remember that the chamfered guide needs to point at the short edge of the bed (the offset from the corners is not the same from each edge, hence the need for two mirrored versions of this jig).
Hope this helps someone! :)