The conventional method is to bolt the handle to the seat base. That’s how the GB4 handle, Reeder handle, and several other handles are designed to work. The problem with that method is that it requires a stiff seat base otherwise when you pull up or push down on the handle it flexes the seat base which will eventually cause it to fail. The handles that bolt to the seat base really need either a strong metal stiffener in the seat or a nice stiff and strong carbon fiber seat base.
The alternative is to have the handle attach to the seatpost instead. Or you could have it attach to the brake post thingy that’s weleded to the seat rails.