have you tried Febo mixers? About three times the size of chums and a lot heavier. They take about an hour to go soft right through.
No I havent caught on them ----- yet.
Re plastic mixers, the counterwieghted ones SHOULD work the best, but I've yet to get the balance right after a lot of fiddling. Personally I find bread a better hookbait enyway.
Back in may when we had the first bit of good weather up here. I had 10 in 3 hours to 16.5Ib some of the lads there had been on since 6 in the morning and i turn up at 5.30 in the afternoon and caught they had all but blanked 1 fish out of 7 anglers. The plastic corn was taken solidly without essitation. The best is that every time you re-cast to a new fish you did not have to rebait.
Iv'e had a couple on the fake chum mixers, but as RIK says, the balance has to be spot on to get the benefit!...
Feed normal chumies to get confidence, then drop in the fake, I use a size 10 ESP hook, any bigger and the whole set up tends to flip over cos it's top heavy....