The nozzle ( tongue) that a standard Crossflow is hydro dynamic (like an airplane wing) that is difficult to make.
There are other arrangements to accomplish the same thing but not in continuous basis, instead the flow is controlled in steps, like 1 / 4, 1 / 3, 1 / 2, 3 / 4.
The width of the cross flow is small, so let me ask, what is the power that you are generating ?.
Also, are you having problems with the water volume and need to adjust the volume due to water level restrictions ?.
Inform and may be we could offer different solutions to your problem.
Regards
Nando