How to waterproof a hole in the vinyl deck to fit a downspout

I am wondering if anyone might have some suggestions on solving this drainage issue with our vinyl deck.

We used to have 2 separate decks but upon rebuild we have them joined with a narrow walkway. There was previously a downspout where the new walkway currently is. Initially, we were going to redirect this downspout to the other side of our roof gutter as our deck builder didn't think having the hole through the deck would look good. But after having a roofer and a gutter company here, we were told that due to the current slope of our roof, redirecting the downspout to the other side would not work without the risk of having our gutters overflow. We have temporarily attached a piece of downspout across our deck that then drains down into river rock for the time being, but of course, it cannot remain this way.

As a result, I now have to cut a hole in our deck for the downspout to fit through so that it can reach the storm drain below. The deck surface is the Deksmart vinyl product. I want to make sure that I do this as least invasively as possible while also being accurate. The last thing I want is for water to seep under the vinyl and potentially cause bubbling or rotting, so I want to shield the exposed edges in the subfloor once I cut out the hole. Almost all water will be inside of the downspout, but I want to protect the subfloor in case some water on the deck itself runs into the cut-out. I have attached some photos for reference.

Gutter Fix 1 – This is the current layout, as well as the proposed plan outlined in red.

Gutter Fix 2 – An alternative view from under the deck.

Gutter Fix 3 – This is a mock-up of how I was hoping to potentially protect the subfloor edges (the green tape represents vinyl decking that I would heat weld to the existing vinyl.

I have also been searching online for options, but I'm not sure that I have found a viable solution. Our gutters are 2×3" rectangles and the only PVC coated drain openings I could find online are 1.5" or 2" circles.

Could anyone suggest a product or PVC insert that will work with a 2×3" downspout? Or a better way to heat weld the vinyl to protect the cut-out opening? Or any other suggestions or ideas on how to accomplish this?

Any help would be greatly appreciated!