I think it is bad pratice to seal drain flaps up.Ihave a MK.11.SD.fitted with drain tubes (five inch plumbers pipe ,one each side through rear hatch and comes out under rear seat) round flaps at rear instead of rectangle as in world held shut with bungie the benefit with round is I fitted five inch hatch covers under the seats screwed on at night for sleeping
Crewe stands on fore deck to keep drains out of water to do this.
result dry boat for sleeping and still sails dry in a few yards after a capsize.
I have secuimar from experience ,If you invert in a big swell each wave rams the mast into sea bed.It breaks.
Suggest if you just cruise you have rectangle drains changed to five inch circles and fit hatch covers on inside.