Hello, it's my first post to the forum, what a wonderful resource!
I have a 1975 DS2, hull #7804. My aft cleats were very loose, and I wanted to replace them. I searched high and low on the forum for aft inspection port install topic, but couldn't find any, so I'm sharing my experience for others. Hopefully this is the right place for this post.
I drilled a series of small holes to cut circular holes for 4" inspection ports, port and starboard, in the vertical face of the seats (not the seat backs).
Thankfully there were no structural stringers in the area. I used a chisel to cut out the foam under the seat so I could get my arm in and up to the cleat backs.
There is a stringer which runs the length of the boat along the deck; the cleats had originally been installed right at the edge of the stringer (on both sides), so the backing washers were not fully seated on the underside of the deck. Perhaps that's why they came loose in the first place. You can see here the holes where the cleats were originally, as well as the new holes I made, along with steel backing plates for better holding power, in a flat part of the underside of the deck. I cut stainless steel plates, and used the cleats as guides to drill matched holes in the backing plates.
Trying to perfectly place the new holes, I stumbled on the idea to lay the backing plate on the top side of the deck and shine a light down on it. Then, I stuck my phone inside the inspection port and took a photo, pointing up towards the deck. The shadow of the backing plate shows where there is room between the stringer and the deck/hull connection. The photo doesn't do it justice, but it worked very well. Cool, huh?
Then I re-installed the cleats with backing plates, washers, and lock nuts. They are solid as a rock.
Then I installed the inspection ports as usual and closed it all up.
It worked out well, except for dropping a wrench under the aft cockpit sole.