DIY help for dress that’s too big?

I bought this dress last year and have since lost about 30 lbs. This pose was kind of awkward so it’s hard to tell; but please observe how the back has extra fabric. It was possibly the most expensive dress ive ever purchased so I want to be able to wear it. Without having a tailor take it in, is there by any change a product I can buy to make it fit smaller? I’ve perused around on Amazon and it seems there may be a few DIY options for this but not totally clear on which item I need. Any help is immensely appreciated!