Use paper towels or cypress (the red stuff) as bedding and keep it very humid. Spray daily and try to keep humidity up to about 65-75%. I say use these for bedding because they will not mold in the humid environment. Or.. You can make a humid hide by putting moist but not wet cypress bedding in a hide box.
Your vet should have given it a few injections of fluid under the skin if it's that dehydrated. Also a daily soak for a short time in warm, shallow water will help.