I've designed these 3/6 way joiners for 21.5mm Pipe. They can be glued or joined with an M3x30mm Bolt and M3 Nylon Locknut. These specifically fit 21.5mm pipe that can be found at B&Q and Screwfix in the UK. 

A perfect addition to your whoop track, you can make cubes of infinite sizes, standard gates with flags on top and much, much more.