Here's what I ended up with for the final model. I solved the flow issue by using drip irrigators to water the pads. Each dripper flows at 1 GPH so the pads stay wet with a minimum amount of water. I kept the baypass since the hoses leak at the joints if I took it out. I think it helps the pump prime too. I used hardware cloth to hold the pads in place as well as screen the inside of the fans (I kept sticking my finger in them). The pump has a hard time priming and I have to unscrew the connection at the pump to let it prime if the lines are dry. Its fine if there's already water in it. I used a dremel tool with a spiral cutting bit to cut the plastic to prevent fractures.
I experimented with the number of pads. I filled the box up with pad material and ran a dripper line over it. It didn't seem to cool the air anymore but it did decrease the amount of air coming out. I settled on just the two pads as shown.
The container I used is a file box from Office Depot. I probably should have made the pads smaller so it could hold more water but it will run at least 4 hours with the 1/2 gallon it holds now. I left out the louvers since they didn't really do anything. I think the louvers were supposed to help with leaks and this one doesn't leak like the mockup did. I can stick it inside the trailer on a pull out shelf. It fits nicely in the sink so I have a place to stow it.
Does it work?
Well, cool air comes out of it and it cools the inside of the trailer when I stick it inside. We'll see how well it works the next time we go camping.
Bruce