The problem with attempting to make your own is that the powerheads that you might want to use (budget wise) are not compatible with being turned up and down or on and off to create the wave effect. It's not only in the design of the controller, but it's also in the design of the powerheads. If you attempted it with a "standard" powerhead you'll either torch the powerhead or you may find that when it's supposed to turn on it doesn't and then you've got more problems.
Save your money and find one used.