Pour some bleach in, fill with water, and let it sit for a day. Done this with Engel bait coolers that somebody forgot to empty a few times and gets all the smell out.
I have been using oxyclean when my engels get funky. It is amazing how clean and fresh everthing comes out. The net inside will look like the day you bought it.
I put half a scoop in the bait bucket and fill it with hot water, shake it and let it sit for a day. After that just rinse it out a couple times and its amazingly clean.