I own two actually. A friend was downsizing and gave me his one. Maybe use them both. Octave down would be fun with the rubbernecking. I sometimes used both switches. Right foot on oscillation and left foot on rubbernecking. Keep playing an let rubbernecking lose and the repeats are an octave down.Anyone who owns a rubberneck is entitled to my utter respect! This pedal is such a jewel of engineering...
To build you could have fun with an octave down in the feedback loop of the delay, maybe the boss OC2, I think it's a pretty straight forward one.
If you're planning to play around, the deep blue delay is quite fun, change the frequency response, add delay lines, parallel, cascade, series, or even drop another effect before the feedback.
All great suggestions so far.
I'll suggest Prunes & Custard for some quintessentially-unusual d(ess)irt... PPCB Curds & Whey
A Vox Repeat Percussion modded to go into pseudo ring-mod territory ... PPCB Woodpecker
GLITCHEE would be good — MBean's mashup of the EQD Bit Commander (octave-synthy) and Seamoon Funk Machine (quackery at its duckiest).
What I really came in here to suggest was the Oberheim VCF, otherwise known as a Filter/Sample/Hold — PPCB Minnow