Marine plywood / epoxy would not be neutral buoyancy - although it probably wouldn't be of top concern in a lost rudder situation, Handling would be very different from a rudder with proper buoyancy.
The force on a hard over rudder at hull speed is measured in tons. Hose clamps and a boat hook attached to a barn door will be very exciting, and tragically, almost useless.
The best bet is to have safety straps (they can double as stops) external to the rudder. If it falls off, you can get it back.