Yes all the DC controllers switch the negative side. You could possibly convert the controller output to a positive switch with additional electronics (such as a transistor).
Other than that use a mechanical relay as they work if either positive or negative is switched. Should be easy to find a 240v 2amp rated one.
Another option, if your solid state relay only needs 5v and a few milliamps (to activate the relay), you could remove the mofsets from the 3 channel controller and use the output signal directly from the microcontroller. They will be a 5v+ signal, upto 20ma I think.
Other than that use a mechanical relay as they work if either positive or negative is switched. Should be easy to find a 240v 2amp rated one.
Another option, if your solid state relay only needs 5v and a few milliamps (to activate the relay), you could remove the mofsets from the 3 channel controller and use the output signal directly from the microcontroller. They will be a 5v+ signal, upto 20ma I think.