I'm not sure this is the answer I'm looking for / what I'm going to choose to use, but I did find these products last night, but can't find any studies or reviews of how well they work:

All are going to cost me more in shipping than the actual product costs, so I'm not sure how cost effective this would be / am still looking for alternatives.

My contractor started installing a soffit before I was ready, so I ended up using expanding foam as a way to insulate isolate my ducts and shaved it to size before it was fully enclosed. It worked, but I'd advise against using regular spray foam (like I did) as it will expand and cause your ducting to shift out of place.

If you go this route, choose the window foam variety which is designed to be "minimal-expanding" as to avoid warping and shifting your ducts (both polyurethane and the latex varieties should work).

Alternatively, you might try a 'no sweat' kind of product I found while previously researching this, but why this answer is currently ranked at -1 as I couldn't find any studies or reviews of how well they work: