Make it as long as possible and, if positioned between a wall and cabinet, cut the ends to be a tight fit.
THE BENCHTOP is a made of a piece of 1800 x 600mm leftover melamine sourced from a local kitchen cabinet supplier with the edging strip thrown in for just $20.
THE SUPPORTING FRAME is made of 19mm pine. From a 42mm board cut two 450mm bench cleats, from 90mm board cut an 1800mm bench support and from 70mm board cut two 550mm wall cleats with 25 x 25mm dog ears at the base.
Attach 500 x 400 x 25mm brackets through the wall cleats and into the wall using screws.
TIP Buy metal brackets for about $17 each from a hardware store. Buy melamine new from a hardware store or get it secondhand from a demolition yard.