Hi Wafflycat
Super idea!
I will certainly be speaking to my Wife about saving the fabric conditioner tops.
In the past I have used polystyrene pieces from packaging on washing machines, T.V's etc but the most useful and cheapest I find are the plastic drinking cups from the tea/coffeee machine at work. A quick wash out and they can also be used as pots for potting on small plants / cuttings. (Making sure you have punched holes in the bottom first

).
Best of luck for the coming season.
Regards
Eddie