I use courgette. I cut it in 2 then slice down the middle, put a fork in one end between the skin and the 'meat' and then put it in.
The 2 commons (gone next week) finish it to the skin and sometimes even the skin is gone by morning.
They will eat it when you're not looking because they feed voraciously at night when you're asleep.
(If you turn the lights off at 11-30 and watch from about 12pm, without letting them know you're there, you might see them feeding on it, and then dart away when they see you)