Hi,
I ahve a push button throttle interface designed for RC ESCs but this is not what you need for a normal ebike controller. It slowly ramps up the throttle, and does other fancy stuff like limit power, cruise control, ebike stats, data logging, blah blah blah. But over kill for what you need. I do really like a button throttle on low power setups. Makes it easy to install on drop bars, or flat, and easily put multiple buttons around the bars as needed,a s they are cheap and easy to put in parallel.
The ebike throttle is different, the three lines are Ground, 5V, and signal.
For a typical hall throttle the signal wire should be ~0.8v at zero throttle and ~4.1v at full throttle.
So the simplest push button throttle would be a simple voltage divider resistor network such that when the button isn't pressed you get a 0.8v on the signal wire, and 4.1v when it is pressed. I think Kepler has posted a schematic at one stage that does this. It is really very simple jsut a few resistors and a push button.
I would personally never have a toggle switch that can stay on, as I find it too dangerous.
The other issue that you may need to resolve is if the transition from zero to full throttle is too quick. With a twist/thumb throttle it does take a small amount of time to twist from 0 to 100%. This smooths out the transition. With a push button this transition will be instant and may not feel right on the bike. But I would give the simple on/off button a go, and only tackle this problem if it is real.