I measured the 3-fret distances on my old Folkcraft dulcimer and then made the triangular Fret Rocker shown from a piece of scrap aluminum I had on hand. 

I tested it against the Folkcraft and found that two frets were high, but not enough out that I could hear any problems.

If I was making another in that size I would increase the short side to 6 cm.

The size of a fret rocker must be appropriate to the VSL and scale. I may someday make another rocker for my 24 inch VSL instruments. Sides 10 cm, 6.5 cm and 5 cm, maybe.

There's nothing wrong with a 4-sided tool, but it is a bit harder to lay out than a triangle.

The aluminum was a thickish sheet from an old award plaque, not a bit of house siding. I roughed it out on my electric coping saw and belt sander. I did make sure to refine the straight edges by putting fine sandpaper on a cast iron plate and sliding the edge back and forth along it.

A "fret rocker" can be any straight edge which spans 3 frets, but not 4. The "special tools" sold for the purpose are metal plates with 4 sides of different lengths, but you can use 4 or more separate objects as well.

If you explore the guitar makers' websites you will find suggestions that a credit card, small machinist's square or "ruler", etc. are suitable for different places along the fretboard. They don't have to be metal.

You can refine a straight edge by putting fine sandpaper on a mirror and sliding the edge back and forth along it.

Stewmac sells a tool called a fret rocker.  The idea is that you slid this tool along the frets.  If the tool sits flat all the fret are at the same height and life is good.  If the tool rocks a bit, you have found a fret that is higher than the other frets.  This tool works well on the higher frets, but not as well when you get below the five fret.  Has anyone found a tool that has the same rocking motion below the five fret?  (The three fret doesn't count.)