Oh, I'm sorry to hear this.
How cold is it where you are? It's hard to know, but it may be better to leave her outside where she has more fresh air, but if the mold is the cause of the sneeze and mucous, she will need to be kept slightly warm.
You can try the Tylan50 to see if that makes a difference. Weigh her so you give her the proper dosage. Tylan has a wide range dosage (10-40mg/kg), usually I would start with the mid-dosage of 20mg/kg which equals out to .10ml or cc per 1pound of weight, give it 2-3times per day for 5days in a row. I would also give the Tylan50 orally so you avoid injection site necrosis.
You mention you are replacing the box? So are you bringing her back inside?
What type of water container are you using for her? What I saw someone doing and thought a neat idea was to put the water container inside a small shallow dish (like a baking sheet) Anything to help catch spills. You could also try putting a puppy pad under the water container, but you will still want to clean it up very regularly to help prevent the mold.
Just a thought here - I use a wire dog kennel for separation and also have a small hospital/brooder/grow out area outside, so I just use something like kennel cups or "coop cups" that hang on the wire for water/food. Kennels can be pricey and a box does work - but you may want to keep an eye on craigslist or local ads- sometimes you may be able to pick up a kennel for just a few bucks, people move and don't want to transport all their stuff.
Keep us posted.