Keep in mind that with flush hatches, the amount of hold-down power required to effect a good watertight seal is surprisingly light when done correctly. The natural tendency is to increase the pressure when leaks occur, but it doesn't ever seem to solve anything, and actually causes other problems like pulled out padeyes, etc. So you don't need rigidity so much as you want accurate work. So take your time and get it right, that's the most important thing.