drill small hole
in center of bulkhead. About 1/8 is plenty. My day hatch is mounted to rear most bulkhead. That hole is bigger to fit 2 mm line.
I have seen some very minor water intrusion after filling cockpit with water.
1/8" is much larger than necessary
There’s no need to go anywhere near that big. I use 1/32" holes in all my boats and they work fine.
Put a loop of bungee cord in the groove
Then run light cords from the deck lines or fittings to the bungee. This method has the dual function of tethering and improving the sealing of the hatch.
If you vent the bulkheads…
…you eliminate the need to remove the hatches for transport. VCP hatches will not come off unless pressure builds up behind them and even then they generally stay in place. Reducing sun exposure is still a good idea, but keeping them coated with 303 substantially prolongs their life.