I use quite a lot of the 2.5 litre polythene plant pots but - until today - always found them to be a real pain to fill. Being basically black plastic bags makes them awkward.
Try this. Get an empty A10-sized tin and fill that instead - you can even use it like a scoop. Then slip the polypot over the end like a ... well :oops: ... like a polypot :oops: . Turn upside down and remove the tin. Simple.
Hotels and restaurants use lots of A10 tins. They are used for soups, apples and lots of other things. They'd hold approx 7lbs.