No no no, hold on, back up ahead. You don't need 25'. You want an expansion chamber in there, or you're not going to get anything out of it.
I tried some of that flex pipe, and it's a leaky mess. It sounds OK but doesn't hold up.
Try a muffler shop first, they might have a piece of scrap that you can try and see what it does. Another option is the reefer exhaust pipe, but that is very thin and has to be clamped.