I think some sort of foot box on your over quilt will help it stay in place.
It wouldn't need to be insulated, I guess. Insulated or not, something to form a pocket to slip over the foot of your down quilt would be useful. It could be either a foot box or just a strip of material several inches wide. The strip would form a sleeve for the down quilt to slide into.
If there is not enough material, maybe a strap or something. The strap could either run from one side of the over quilt to the other with the down quilt slid in or the strap could connect the overquilt to a sleeping pad. The overquilt would be held in place relative to the pad and the down quilt slipped in.
I think working out some way to keep the overquilt in place would be worthwhile.
I think I would make an uninsulated box, if I had the material. I might make it 12" deep and see how that works. It wouldn't have to be tight or 'form fitting'. Even a little loose, it would hold the overquilt in place. (if the feet are cold, I might insulate the bottom/foot-bottom of the overquilt box, but I don't think I would worry with insulating the ground-bottom side of the footbox...)
It might also be a good idea to think of a way to keep the top part of the overquilt from sliding off. Some sort of strap or something for that could be added later, though, if it is needed. Maybe the old quilt already has a strap.