You can buy a mounting bracket, but it might be hard to find one that works for your boat and/or your trolling motor.

What I did was I took my boat and my trolling motor to a welder, I explained to him exactly what I wanted, and he welded me a perfect mount. You must take the motor so the welder knows the base dimensions and where to drill the holes for the bolts.

This may cost you between $100 - $150, but its well worth it.

The only downside is that once you have this made, you will only be able to use that particular brand/model of trolling motor. Get a different one and you will need to have a new mount made.