I quickly designed it before finding out that others tried to solve the same problem, like https://www.printables.com/model/327021-karcher-pressure-washer-trigger-lock for a Kärcher that may also work for Parkside. Still it works perfectly, is quick to print in PLA without supports.

Of course, there is some risk involved when using it, but having to squeeze the trigger for several hours also hurts your hand…