You can make self watering flower pots using a 2 liter soda bottle by cutting it in half at about 4 inches from the bottom & flipping the top over and placing it into the base. You can also stack two 5 gallon buckets for a larger SIP.
This 'thing' allows you to connect multiple SIPs together enabling you to provide a water reservoir (such as a rain barrel)and have all of them maintain the proper water level easily. It's currently designed to use 1/4" OD container irrigation water line.
Simply drill a hole in your SIP base and use a dab of silicone sealant between the T Block and the SIP base. The plastic nut is to hold it in place while the sealent cures.
I printed mine out at .2mm layer height with 3 solid layers and 3 perimeters with a 10% infill. I wanted a good solid shell so it would be waterproof with a minimal plastic usage otherwise.
[UPDATE] This is now parametric to allow you to change the size of the container it will attach to. Just key in the diameter of the planter.
Soda Bottle = 100 ~ 4.5 inches
5 gallon bucket = 254 ~ 10 inches