just to clear this up RHS = Rectangular Hollow Section. you need square box section also called SHS = Square Hollow Section.
ERW = Electronically Resistance Welded. this is one of the ways RHS and SHS are made.
What you want to order is SHS ERW bright mild steel 16gauge 25x25mm. which in simple terms is a shiney square metal tube with a weld all the way down one side of it.
Simples
