I have a set of 8x10 mm power inductors coming my way - in values 10uH-10mH - so enough to try. They're still pretty cheap, the I-shaped type. Good for prototyping; for PCB design though I'm more inclined to use SMD parts if available.
I also realised my original schematic is wrong, I should put the inductor in the lines going to the buck converter, not towards the pumps. Then at least my digital part is properly shielded from the motors, pumps, air pumps, fans, lights, and whatnot. That also means the current through the inductor will be a lot less, more like 200-300 mA peak (and less than half that continuous) instead of up to 5A continuous.