5 second 'slow on' circuit

I'm trying to build a soft start circuit for an electric clutch (riding mower type). So far, I can only get about 1 second or less to full power. I tried this circuit less the Zener and LED.
Voltage is 12 volts DC, load is 5 amps and using a IRFZ48N transistor.
I've increased the capacitor to 15,000 and played with resistors but nothing seems to help much.

I know I could do it with a MCU (and have) but would like to keep it simple if possible.

Would appreciate if someone could point me to a solution.

Thanks
Aaron