These telescoping box supports are similar to the Nvent Erico CaddyTSRBS1625.  They adjust from 16-25" and can support all 4 square boxes.  For Use With Electrical Boxes and Mud Rings, Ideal For Supporting And Positioning 4 in Square And 4-11/16 in Electrical Boxes And Mud Rings Function, Open Design Allows Boxes To Be Positioned Anywhere Inside The Opening.