2 weeks ago I had to access 3 different sets of mounting bolts that were totall inaccessable. They make deck plates in sizes from 4 to 8 inches. These are screw in inspection ports that have an outer ring that bolts to the deck or bulkhead surface and has a hole from 4 to 8 inches in the center for access. The cover screws in to form a water tight seal.
They are cheap, work very well, are fast to install and unknown to many sailors. I got mine from West Marine and recieved them in 4 days.
Ray