The Dytone Tremolino Tremolo Pedal allows you to continuously adjust the waveform from a sinus to a rectangle and choose between 2 speeds easily. The fourth knob (xxx) will give you a further opportunity to influence the modulation.
Approx. 4 minutes with 2 speeds, various settings of the knobs, but none full up.
There are lots of resources left for more freaky and extreme sounds.