a bracket will not help, it will just move the CG back and the boat will rest more trimmed up at the bow. A bracket might make it worse. This all assumes you want a self bailing boat. If you dont care about self bailing you can just close transom, drain to the bilge.....this is not safe though
You need to reduce the weight in the boat and/or redesign the drain system.
For a boat of this size, its almost impossible to have self bailing with a 150 and a fishing load (people/gear/gas) and still have safe cockpit depth. Raise the sole, and it will bail, but you risk loosing someone overboard. Whats important is that it self bails at the dock !