the perfect vibration will depend of diferent factors :
motor type
motor attachement
type of counterweight (size, shape)
motor placement
board attachement to the frame
voltage
It's difficult to find a perfect answer, I believe. On my own board, I made a lot of experimentation, testing diferent motors and setups (regarding for the factors I gave) before I find a good final setup (with only one motor, BTW).
You can have a look in the tips forum, I present my board construction. Even if it's not THE board making rule burned in the stone, it can help you on your own experiment.
The best thing, since you're in USA, is to find a kind board maker to help you in the materials choices and crafting method
