It is mandatory that hatch boards and washboards to shut the main companionway are retained permanently by some means - typically a strong lanyard. If these vital pieces are lost or defective, then seas breaking over the stern will jet below and quickly fill the yacht. They are thus critical to the yacht's watertight integrity and it is essential that they are strongly fixed shut. Many simple latches commonly used are not sufficient in themselves and should be padlocked at sea. Hatch hinges and latch fittings should be through-bolted rather than screwed.
Good seals are vital on locker lids and hatches.
As a guide, use 10 x LOA, best adjusted to period of seas. 3-strand nylon anchor warp is suitable. Some weight could be provided by a 10m length of chain between drogue and warp. The main requirement to survive capsize is that the drogue line is always held sufficiently taut to prevent the yacht yawing beam on to the sea. Try to keep the drogue in the second or third wave astern.