I'm only on my first batch, and haven't even tasted it yet, so take my advice with a grain of salt. I'm doing a water bath for fermentation temperature control too, but I haven't had much of an issue keeping temperature cool and under control. Where I differ from your method is in a few areas:
1) I use an Igloo Ice Cube cooler as the fermentation tub. You want the 60qt (90 can) model, which comes with wheels or without.
2) I don't use evaporation cooling at all. I fold 3 or 4 towels in half longways, and lay them across the top of the cooler in such a way that only the neck of the carboy and blowoff tube are protruding. This, along with the cooler, acts to maintain lower water temperatures much better than a plastic tub would. There's another homebrewer here who bought a sheet of rigid foam insulation from a hardware store and cut it to fit the top of the Igloo cooler, and cut a hole in the middle for the carboy neck. This would work great too.
3) I also use ice packs, but this is my only method of lowering temperatures. I'm fermenting in the low 60s, typically around 63-64. I haven't had a problem keeping it at this temperature. During active fermentation I had a short spell where I used 2 ice packs at a time, but typically I only need one ice pack and exchange it every 6-8 hours. The ice packs I use are 16oz.
4) I add a small amount bleach to the ice bath water. I haven't had any signs of mildew yet, and I certainly wouldn't want mildew building near my fermenter. A couple of my towels did kinda fall into the water a bit at one point so I removed them and re-washed them.
Ambient temperature in my place is around 78 degrees. I do think this method would become a bit of a burden over the long run, so I will eventually migrate to a fermentation fridge. Here are a couple of pictures:
This is the thread for my first homebrew batch.