Product review (4 stars): This is a great starting point
I considered this a starting point. I assumed it would be flimsy based on other reviews but I ordered it anyway. When I got the item, I immediately reinforced it with duct tape, sealing  any potential leaks and reinforcing the ring part.

I used hard ducting cut to size. The one end fit smoothly into the ducting, and I sealed it well with duct tape. For the other side I had an extra pipe joiner that was almost the perfect size. You can get these at Home Depot for about $6. It includes a rubber grommet that has a butt seal inside, a metal band and 2 integrated hose clamps. The two ends met seamlessly and there are zero air leaks.

The unit works great. And we have even run the dryer empty for supplemental heat.

Only complaint is the lint catch system could be a little better, but if it was too fine it would probably blocked much of the air flow, as we use a dryer sheet inside the air filter as well. We prefer to just clean a little lint off the floor from time to time for the enhanced heating.

If you know what you're doing, this is a great little starting point.