You can use the CAB template contract here: https://drive.google.com/open?id=0BxLFp1MRwjgqb3lHOFlCYlNFcDAzcVFCX3BEQ25wSmhrb3Vn, along with a verbal committment. Some folks may need to run it by their legal to give the once-over, but by and large its a handshake agreement.
Pro tip: Try pushing to include that you can use their logo on your site - there shouldn't be much pushback if you clearly indicate that they are part of your Customer Advisory Board.