I'm just in the middle (literally, I just mounted the plate 30 mins ago) of adding a cold plate to my drink cooler on my 2002 330 express. What do you need to know?
I'm using a Webasto air cooled unit, bought from defender. They sell kits that are plate+compressor and have different sizes. You can bend the plates to an L or U shape, carefully....
While I'm sure technically this would work fine retrofitted in a fishbox, I would be a little worried about having the plate exposed. If you were serious about it, I might be inclined to go a layer of styrofoam, then a plate fiberglassed in.. but I haven't seen how the factory does it...
If you need a lot of cooling power you'll probably be looking at a water-cooled unit.