I've designed a collar that tightens around the top of the spray can using a zip tie or M4 nut and bolt, this mounting system coupled with an internal groove allows the handle to be perfectly secured to the spray can, without the risk of slipping and also makes the model really durable. To increase the durability further, the trigger is to be printed horizontally, and attaches to the handle with a M5x15mm bolt that acts as a pivot and a M5 nut.

Notes:

Print settings: 0.2 resolution, 2 or 3 shells, 15% infill, supports on (touching build plate, only needed in the collar area) and i suggest, if your slicer allows it, to print the collar solid, to make it tougher, example photo below.

The handle in action: