If you are using a bilge pump I found it doesn't work very will if it is not covered on the bottom, it agitates the water more then it actually pumps.
Also, I 1/4 inch lines directly from the pump (with adaptors) out the lit of the cooler, then sealed with silicone. No need for fancy pass through fittings, we're talking water here, not acid, and being on the top of the cooler it didn't leak at all.
Silicone sealer on every screw hole kept it from leaking at all. The dry break fittings from mcmaster worked very very well and are simple to install. Also rout the return line so it goes directly into the ice-end of the cooler as well to make it work better. With the exception of the dry breaks, all the fittings needed I found at west marine and home depot.
I used a weather pack connector for the electrical connections so the switch and wiring stay in the car. Wires were also routed through the top of the cooler and sealed with silicone, just remember to leave enough slack in the tubing and wires and you'll be fine.
soft plastic sheet or aluminum flashing/aluminum sheet is fine for the inner baffle, keep it off the bottom so the water can move freely underneath while the baffle will help prevent slosh and keep the block ice/water bottles...etc from moving.
Remember the bigger the block of ice, the longer it will last. We had no issues. The water doesn't need to be super cooled to provide the needed affect, its more about maximizing the tubing surface area in the shirt to provide for as much heat transfer as possible rather then squeezing out a few degrees. We found the big ice containers provided a great balance between cooling and ease of use/changing.
Also, we used a large cooler full of dry ice (luckily I have a connection there) to freeze/re-freeze. I filled it up thursday afternoon before summit and it stayed frozen until sunday morning, of course only open when needed. Water freezes incredibly quickly at around -100 degrees ;-)