Does such a tool exist?

[mr haynes]

Lumps of wood and yer brakepad spreader for micro adjustment

 

[Parm]

If you can wait for delivery from China, these chaps will be perfect, nice n cheap too

From the Temu website, I searched for board props

 

[Kram]

If you can wait for delivery from China, these chaps will be perfect, nice n cheap too

View attachment 506410

From the Temu website, I searched for board props

Hang on, thats a caulking gun with a bit of plastic stuck to the back.
A poundland type could likely be repurposed, as there not any good for caulk!

 

[bricol]

I've used these before - came with a couple of cheap spirit levels with laser beams from Makro a long time ago: