Idea is to glue this to 20mm threaded PVC pressure pipe fitting.

Then make a long PVC pole (using glued elbow joints to create the 180 degree elbow to get the fitting up to the gutter.

My gutters have a gap between colourbond/tile and gutter of less than 40mm so this is designed to slide lengthways into the gutter, then you rotate it 90 degrees.

The other end of the PVC pipe can be a screw and quick release gardening hose adapter.

Print one for Left Hand gutter flow and one for Right Hand gutter flow.

The plenum should provide even water pressure out of the narrow 3mm slots.. producing a flat water jet suitable for blasting along the sides of square profile gutters (115mm and wider).

The 2 pieces are meant to be glued together. Pressure strength of the ABS isn't so critical as the water is designed to flow out of it.