After building the first prototype I collected all my old flashes I got over the years and had four more slaveflash-triggers to built.
I already figured out an PCB-Layout for the SOIC-version of the attiny 24. But the biggest parts in the design are actually the hex-switch and the optocoupler, so it would make no sense to substitute only the microcontroller by a smaller one. Instead I replaced the hex-switch by a learning mode in software and also replaced the power source.
By replacing the 7805 voltage controller by a button cell with 3V, the power consumption was reduced from 2mA to below 0.5mA without the LED on.
So here it is: Let's got started!