If you are unfortunate enough to purchase Orbit Flex sprinkler risers, they will eventually fail. The design is not meant to handle pressure, so the flexible poly tubing inside will inevitably pop off. 

Inside the tube connector is this piece (3D modeled here, also shown in photo in black) which is essential for making the tubing stay put. Once the riser fails, this piece will probably either shoot up into the air (never to be found again) or be buried deep down in the mud.

You can print this part out as a spare. I didn't bother to recreate the barbed ridges or add a rubber washer because I simply wanted to super-glue the tube in place rather than have it pop off again. Tested in the field and its been working well. Print in PETG over PLA for the additional water resistance.