alisonn, am assuming that the back has multiple cushions. If there is just one back cushion, get a large piece of whatever thickness of foam is OK with you- but not thicker than the base cushions. Wrap this with poly batting.

For multiple back 'fluffy' cushions on a sofa bed I once re-stuffed by wrapping upholstery batting around an inner core of 1" thick dense upholstery foam. This batting is a stabilized variety, 100% poly, and does not shift. To give the pillows a 'pregnant look', I would cut the batting in staggered layers and spray glue them to both sides of the foam and then final wrap. I always covered the cushions with a slipcover of lightwt muslin and then made the final cover.

In the 80s and 90s, I could only buy these materials thru an upholstery supply house in Baltimore area. Have heard that Joanns now has a wider range of foams, but I cannot deal with the fragrances sold in that chain (certainly spend my discretionary sewing allowance elsewhere!)

If your cushions have high quality filling, you might un-assemble them, and see if you can make a central slit and insert a slab of foam and loosly base...sort of like filling a pita bread or cream puff...if you cook. My upholstery shears have long blades, but am sure reg sewing shears might work with patience.

The first pillows I 'reformed' were on an expensive sofa, but the poly pillow filling was the worst and recycled to stuffed toys! Good luck...