Do I need a power supply unit or just a voltage header?
You can wire the 240VAC heaters in parallel through an SSR or similar. How you do it is up to you, but a terminal block, spade connectors or even individual connectors (all properly insulated and rated, of course) might be options. I suggest fusing each heater individually.
You probably should include a series thermal cutoff or even two, as 2.8kW is a significant amount of power and might be enough to start a fire should the SSR fail 'on' or the sensor become dislodged. Since controllers are so cheap, you could consider a redundant sensor and limit controller, but simple mechanical thermal cutoffs are good to have in any case.
You would want the pads to be on a surface that spreads the heat (eg. thick-ish aluminum plate) and the sensor probably near the center.
The controller will adjust for minor variations in heat demand and mains voltage.