I have the water kit and have used it about a dozen times for water hunting. Toothpick is correct, it is easy to build... I make a lot of stuff and making the Deus waterproof is super easy. However, there is a big reason I bought the Water Kit sold by XP... and that is... it is a $60 insurance policy! Another member and friend of mine (trebelhunter) on here was using the XP water kit and his remote got flooded. XP sent him a new remote, under warranty, no questions asked. Had he not been using the XP waterproof kit when it got flooded, I doubt that would have been covered under warranty. So I decided to just buy the XP kit as an insurance policy incase I ruined equipment in the water. That said, the Aryca case for the remote is WAY better than the bag that XP uses in the waterproof kit. I use my Aryca case when hunting in and rain, and I would use it in the water if it wasnt for me wanting to keep everything under warranty... once my warranty runs out, I will be using the Aryca case in the water.
As far as the water kit goes that XP sells. Its nice and fully submersible. The remote case has a sealed headphone wire that comes out and it comes with waterproof earbuds. So you can dive with it and go totally under water. The antenna wire from the coil to the case its plenty long and easy to attach and remove. Even though the remote case is submersible, it still worries me to take it totally under water. Most of my hunting is waist deep or less, so I put the remote on my arm and keep it mostly out of the water.
If you're going to do a lot of water hunting, I'd suggest buying the XP kit because its a cheap insurance policy to retain your warranty on anything getting flooded. If you are out of warranty or just want to try it out once in a while, then making something is cheaper and easy.