How do I fasten the upholstered back to the banquette frame?

Is the frame already attached to the wall?

Hello Gil! I am not sure why you have the three 2x4's attached to the frame (up against the wall) unless you are trying to make the back of the banquette be away from the wall. You could make foam cushions to put in between the 2x4's-attached to a sheet of plywood.

https://www.hometalk.com/diy/build/furniture/built-in-banquette-tutorial-12329511?expand_all_questions=1

https://www.hometalk.com/diy/build/furniture/built-in-banquette-31596945?expand_all_questions=1

Here's a banquette with the back directly mounted the the wall eliminating the three 2x4's. https://www.drivenbydecor.com/diy-upholstered-kitchen-banquette-nailhead-trim/

Yes

French cleat would work somewhat. The back would just hang there. Always a chance for it to fall from use. Sides of 2X4's will be visible. Should have built a 2X4 frame off the wall, upholstered it, then attached it to the bench frame. Should of came here first (Hometalk) and we would have advised.

how is the uphostered piece made is it just a flat piece of plywood ? can you move the 2 outside frame pieces on wall inward at all and add some wood(just like frame pieces) on back edges of back upholstered part then using wood screws attach onto frame from sides does that make sense.... so you would be dbling up the 2 ends pieces in the back on wall & it would rest on bench piece it looks like you have room to dbl them up. AAhhhh ok let me try again.....cut 2 more pieces like the ones attached to wall, move the 2 outside boards over/inward, then add the 2 pieces you cut to back of upholstered piece you'll need really long screws & construction adhesive,then put it up and from sides screw the boards together it should still look like the 2 end boards Well I hope I could get what I'm thinking across to you & I hope it works

i assume you are covering the wall studs first. How about velcro.

If I am reading your question correctly, the back rest for this piece hasn't been made yet, which may give you a little more flexibility in how it attaches. Were you intending the 2X4s to show as part of the design? 'Cause you could probably use L brackets to attach the back if you aren't planning on covering the frame.