Like this?

Yes, thats exactly what I’m going for but don't want to put all the work into sewing a single cushion cover if its not going to work with a foam cushion.

Are you reusing the old foam insert or buying a new single piece?

It can be done, you just need a single insert to replace the individual ones.

I am planning on buying one single piece of foam and foam is pretty expensive so i dont want to go all in if the sides are going to lift up when someone sits in the middle. From what I’ve read, single cushions work best with down fill but thats too expensive

Have you checked the availability of foam for upholstery? Besides being pricey, there is a shortage and shipping is taking months right now.

oh wow, I did not know this. I just purchased foam for four of my patio chairs but your comment does make sense. I don't think I will be working on this project for a few months so maybe I should be sourcing foam now in case it's on back order.

My single cushion sofa has side clips (2 on each side) to keep the cushion in place. To be honest, I have never used the clips. The clips are attatched to the frame of the sofa.

If you have he ability to enlarge the photo, you will be able to see the clips. Hope this helps.

Thank you so much Ratherbesewing!! that really helps! i was thinking of maybe sewing on velcro but then i couldnt flip the cushion. I can def put these clips in when I reupholster and still be able to flip the cushion

Is your cushion foam wrapped in batting?

The fabric will probably allow the cover to slip on without the foam grabbing and making insertion difficult. The batting is to soften the hard look of the foam. The example sofa looks like it has batting wrapped foam.