The bell siphon is the device that takes water from the bottom of the fish tank and transfers it into the grow bed.
The first thing that comes to mind is a simple water trap (e.g. see this). You can use a similar design, but with a bigger hole and a larger diameter pipe to get the water out of the tank. Then, you'd have a small chamber where the bell siphons.
Making a bell siphon for aquaponics is not as easy as you might think. The first problem is that the shape of your tubing determines how much water will flow through it. You need to choose a shape that allows for more water flow than you need to run your system. If you are designing your own, you will have to calculate how much flow you need to run your system, and then design your tubing so that it can handle that flow.
The second problem is that the diameter of your tubing has to be chosen based on the amount of flow you want. If you have a garden hose with a 5/8" diameter, you can't run a garden hose with a 1/2" diameter through it without clogging it up. There is a point where your tubing will have to get smaller than the size of the hole in which it is installed, and this will make it impossible to run a continuous stream of water.