Sadly I don't think chaff will help you. The piggy-banks appear to operate by Doppler, and thus most likely to be continuous wave rather than pulsed.
The radar cross-section of your average oik onna bike won't be much compared to the metal back end of a Q8, so don't be surprised. A bit of corrugated tin facing said camera would probably be sufficient though, and anecdotally it appears a second rider is sufficient.
Given the choice I'd go more active jamming, albeit illegal, so this is strictly a think-piece.
Provided one can determine the base frequency of the camera, which is a box of tricks in its own right, then there are 2 things one could do.
1. False inject a ludicrously high speed into it, say 240mph. v= df/f*c/2, so 240mph difference to 10Ghz is only 16kHz. Incidentally 35mph comes out at about 2.3kHz. This is also why you can't outrun a speed camera. Compared to c, your pedestrian dawdling at anything less than Mach 3 is too easy to set a receiver for. However the Beak is unlikely to agree the camera was operating correctly when 240mph comes before him from a 10yr old Citroen. Potential drawback that if this is the 3rd time, your car might get Heavily Investigated. And this might be a bad thing.
2. Overload the base frequency, so it can't detect the higher Doppler against the noise floor. Bit harder as you have to be really precise when capturing the base frequency, and correct your apparent speed to the stationary device, and hope they're not doing FMCW, but the payback is that you don't get flashed at all.
I'd recommend against barrage jamming, simply because it's much easier to spot that Something Is Going On when SkyTv, AirWaves and who know what else get all fuzzy when that Honda CRV goes past. You also might be actually interfering with Something Important, and that wouldn't be right.