I went with 1X8 poplar to keep the weight down. All those shanks, faucets, and lines start to add up and the original hinges aren't exactly made to take it. You can add another hinge, but I'm all about KISS. The poplar isn't nearly as nice as something like say, oak would be, but it took a stain pretty well and for half the price of oak, I'm happy with it. Good luck.
Of course weight only matters if you build your collar into the lid so it lifts up for easy access to the bottom of the keezer. I recommend this BTW cause all kinds of nasty stuff builds up at the bottom after a few months and you'll need to clean it occasionally.