Dried beans in a sock or other cloth covering will work for maybe an hour and you need to monitor the birds and warm and repeat as needed, small plastic bottles with hot water and covered in a sock or towel work as well, small hot water bottles will work too. The snuggle lasts so much longer like at night so you can sleep.
If you use a sock with dry beans try wrapping it in a small cage
with a mylar space blanket around the outside as it should capture the heat and reflect back into the birds body and keep it warmer. The mylar is cheap but is also delicate. It lasts better attached to outside of a small blanket .
I am seeing these blankets for people and pet beds with mylar sandwhiched
in between the layers here in the US. This could be a cheap experiment making cage covers cut to size with mylar and soft blanket attached to the outside. Best wishes, Jo Ann