At the base of the 3D printed part you could also use a through bolt with a washer or small plate underneath to clamp on. Maybe use some sort of rubber textured pad on either side to grip so you don't have to crank down on the plastics
My first attempt involved a clip securing the detent into the slot, but the float portion of the lever sits where the detent would have to be clipped in to work-so it didn't have enough room.That is one of my thoughts, just use the slot and just have a small thumb wheel/knob on top to tighten it down with then you can move it up and down to adjust the height, lock it in float or just half way down.
Ideas are what it is all about.
I'd recommend designing the hole into the part so it's properly reinforced and prepared for the clamping pressure.At the base of the 3D printed part you could also use a through bolt with a washer or small plate underneath to clamp on. Maybe use some sort of rubber textured pad on either side to grip so you don't have to crank down on the plastics