There are many DIY balance board ideas out there but it's not recommended to make your own. A balance board or wobble board should be well balanced. When you are standing on it and keeping your balance, the board should be balanced too.
If the weight or shape of the board you make is off, even by just a little bit, it could force you to keep your balance on an uneven surface. Short term, your exercise will be ineffective. In the long run, this may cause injury.