You can most likely find a used 200 for less than you can build one out of plywood. Back when I built mine, the cost was about $300 for the total project. Nowadays, just the wood will cost you more than that.
I plan on starting mine in the spring time. gonna be 72x30x36. got a spot in my basement right next to a closet that is gonna work perfectly. The area is 8 ft wide and am planning on building cabnets at one end of the tank.