Oxalic acid fumes are not a problem. Place the frame inside a couple of large garbage bags. Add your solution. Don't have to add 30 gallons, just a couple. seal the bags so it doesn't leak. Every day, flip the bags and frame over.

When you are done, take the frame out, wash very, very thoroughly with water to remove the acid. You'll need to treat any bare steel immediately to prevent rusting.

Simply leave the bags open and allow the solution to evaporate. Then throw the bags away.