DD (9) has asked for one. She doesn't know if they exist or not so doesn't have any firm criteria I don't think. Ones in Amazon aren't reliably able to deliver on time so I am hoping there would be something either on the high street or supermarkets etc with click and collect a possibility. Am willing to go up to about £15 - I have seen there is a horseshoe magnet in John Lewis which I am holding in reserve 
Many thanks 