This is how i mounted my depth finder for $6.00
I went to Lowes and bought a pipe repair clamp and a flat piece of metal stud plate. Drilled a hole for the clamp bolt to go through and drilled holes for the depth finder mount. Total cost $6.00 The other clamps in the plastic are for mounting the transducer to the trolling motor. They cost $2 at wal-mart.
Link to the repair clamp: Link to pipe repair clamp at Lowes
Link to the metal stud plate: Link to metal stud plate at Lowes