If the vet thinks this is a strctly nutritional issue, I would suggest a bigger more comfortable cage for mama and baby. You can get a collapsing wire cage for 50-60 bucks new if you look around, or try to find a used one. This allows a 360-degree view, so the cat isn't so claustrophobic, but most cages are large enough for a small litterbox and a bed, in addition to a blanket. I borrowed one from a friend to keep Havoc in after his leg surgery. He had all the necessities, but could still see and had room to move around.

May try to supplement the little one with KMR and Nutri-cal too.