Does her w/c have lateral supports at the hips? Thighs? These can be added by your DME if not. If you have them, they may need adjustment as she progresses and they are more comfortable if you insert a piece of sheepskin in between body and support. We also use a Stealth headrest with "wings" and an "egg" on one side of the head, w/ a fleece pillowcase over it for extra cushioning, that we remove for transfers. If she is falling over bc she has lost girth, the arms can be moved in so the whole chair is narrower. That might be the crux of the problem. And depending on where she is weakest, a cervical collar in the chair that is not cushioned in back (we use the black sleep collar when we go out) may help also.
A seat belt or harness could be good if her body shape permits. My husband's doesn't.
The small travel pillows sold at BBNB are very stuffable since their polyfill is more forgiving than couch pillows. If you live or travel near a foam store, I would go there and look around. We currently use foam store purchases for calf padding in the w/c, arm trough padding, arm padding in bed, under the feet in the w/c (which also helps stabilize the body)...so many things. If you don't have access to a foam store, you can order pieces on line though of course that is harder to visualize.
You didn't ask, but a note on the "upright vs. reclining" part -- to put the sling behind him, we have to lean him forward and hold him in position using a hand on his chest, often his back as well. If you leverage the center of gravity, it should not be a struggle but it is hard for him so my son has perfected putting the sling behind the back as quickly as possible. We also manually support the head at other times (e.g. in the sling since he needs anterior support and the cervical collar only does so much). So however you optimize the seated reclining/tilted position w/ makeshift supports, you are going to have to jettison much of it for transfers -- take everything out -- and just do them as quickly as possible.
