You may want to request that such a form be omitted entirely. Plastic bag covers will cause the fabric to yellow. For the safety of the gown fabric, avoid treatments to kill fungi and bacteria. These dry cleaning suggestions apply only to sturdy fabrics and trims. Fragile gowns requiring extra care should be cleaned only after consulting with a conservator trained in cleaning fragile textiles.
Many public museums have textile conservators on staff.
These individuals might also refer you to conservators in your area who would be willing to clean your garments and repair them for a fee. If you choose to buy this service, ask what type of materials and processes are used. A good dry cleaner should use inert, acid-free products. You can also assemble the necessary packing materials and pack the cleaned gown yourself. The following may help you compare costs vs. convenience.