I found the site you were talking about for $17 you get the plans....it says building yourself will only cost about a $100 for one!!!! How about this?
Submersible Fishing Light - Fluorescent - 20' Power Cord and 20' Safety Cord - Green Optronics Lights UW-300G
It's not 4' but you could buy 4 for what it will cost you to build 1!!!!