Cheap and cheerful. 2 nets from decathlon
Under: small net. Ideal for tool kit, shopping bag, legging and it can also store the large net.
Over: large net, what ever tickles your fancy. Helmets, football, boxes, a tray of coffees 2 go.

Both nets are suspended between the seat post and the rear lift bracket. Dismount the rear light and remount after fitting the nets.