Positioning the pressure regulator after the solenoid valve (as shown in the latest documentation) is correct. If you have any issues with the tubing popping off the barbs due to high municipal water pressure, you can use the included zip ties or even hose clamps to better secure the tubing.
In previous versions of FarmBot, the regulator was placed before the valve. However, the pressure regulator we’ve chosen is a spring-based design, which is not designed to handle sustained pressure load. We found this out after several months when the pressure regulators placed in that location began to fail and no longer regulate the pressure at all.
Alternatively, you can also pick up a more expensive pressure regulator from your local hardware store that is designed to handle constant pressure, and install it right at the hose bib. Some of them even allow you to adjust the output pressure.
Regarding a Solenoid Valve Housing: This is certainly a place we can improve the FarmBot design to increase longevity of the system. Right now, you might be able to modify the existing Vacuum Pump Housing by drilling a hole in the top and adding one of the grommets used in the v1.2/v1.3 design. Looks like such a mod has already been done at least once.