Sapphire Plus heat bed support align tool

Assembling the Sapphire Plus, at some point you get to the chapter "Install the Hot Bed Support". The manual states "Fix it to the sliders on both sides and keep it horizontal and not skewed".
What they mean is you have to mount it perpendicular to the lineair rails. If you keep it horizontal and your table is not 100% horizontal then you end up with a skewed fix.
What you need is a simple tool that helps aligning the heat bed support perpendicular to the rails.

I keep a list of problems/fixes that I have with the Sapphire Plus on my website.