How to add a lock/latch to an existing cat flap in an aluminium door?

Caro

We have an aluminum sliding screen door that is fitted with a cat flap to match the pattern of the door.  Unfortunately, it does not have a latch fitted. I would like to be able to lock it on the outside of the door so I can lock the cat inside and keep the glass sliding door open.  Currently I have to close the glass doors to keep the cat in the house.  It doesn't need a latch on the inside as I want the cat to be able to enter the house anytime.  Any suggestions you could help with would be much appreciated. 

 Thanks

Carolyn

           

Noyade

Hi @Caro

I was thinking of a small rounded bracket with a self-tapping screw and washer that at night you turn to the vertical position, allowing the cat in - but not out. I think that's what you are after?

Cheers.