I'm adding every fix to this problem that I can find to this collection. You only need to pick one. blueagent14 and Spconnol's designs don't require any extra hardware.

The stock Bowden coupler on the Monoprice Mini Delta 3D Printer is terrible and will break. Here are some 3D-printable fixes. Some require additional parts. Others (such as blueagent14 and Spconnol's designs) do not require any extra hardware.

You MIGHT have to compensate for the poor dimensional accuracy of your printer and/or the STL file in order for the printed part to fit appropriately. This is explained on this page:

You could also just replace your extruder. Wiki link included, but others may work as well.

Spare Bowden Connectors

Some people use tape to hold the Bowden tube in place while printing replacement parts.

It might also be a good idea to install these on your Bowden connectors.