This took a long time to get around to, but ended up being very simple. The magic gubbins is a manifold made from a short piece of the 1/2" delrin left over from the nozzle holder work. It is drilled lengthways to a size which is a stiff push fit for the 1/4" tubing. There is a side hole in the center the same size to feed to the head. There is a small hole opposite this which is a tight push fit for the sensor. This is stuffed in there to stop the hole for now - it is not wired up. 

The solenoids are 12 from American Science and Surplus and were 10 for $10. 

Pump is from robotdigg,  $10.80 + shipping

Sensor is from banggood, $3.76 for three....

Tubing was closeout sale stock from Orchard Supply Hardware.

Mounting bracket is a piece of carved 2x4 pine/redwood, 3 drywall screws and 2 cable ties. 

This setup is untested. I'll have a go without the sensor at first as I think it will take some software fettling.