Our female desexed cat has been peeing on the couch.

She seems to be very attached to DD, and the peeing occurred after the family had been away at Xmas and now at Easter.

Our vet thinks it's a stress response (presumably to 'her baby' being gone) rather than due to a uti.

Ok so I thought I could deal with it. We've got a Feliway diffuser but it seems to run out very quickly, faster than monthly, so it wasn't working over Easter. So last week when we got back, she peed. But now the diffuser is working and she still peed yesterday and today. Yesterday we were home at the time and DD was napping, today we were out at the shops.
Our couch is permanently covered in plastic and cheap blankets/towels.
DH is losing patience and i am too. Apart from the pee the cat is acting her normal self, but she is a sensitive cat anyway. We saved her from the pound at age 10 mths, she's around 5.5 yrs now.
I don't want to take her to the pound as I know they will probably put her down. But I can't have a cat that pees on my couch constantly and I can't keep the couch plastic wrapped for ever!
I don't really know what I am after but if anyone has suggestions please let me know.
Oh and she's an indoor cat, becoming an outdoor cat isn't an option, nor is locking her in a room as that stresses her even more.