I just bought my house last year and when the company came out to close the pool and blew the air through the lines, we noticed the bottom drain cover kind of displaced when the large bubbles went past it. Over the winter, the pool was closed, then we opened the pool up and found that the cover had moved off of the inlet area. We pulled it up out of the water and found that it was never actually connected to the drain inlet, but instead was simply weighed down by 2 weights and a bit of wire strung through the holes in the drain cover.
Well, simply putting it back wasn't an option because it didn't seem safe, but also because the wire they used was copper and had thoroughly corroded. So I planned on attaching the cover properly to the inlet to make things safe and found that the inlet does not have any screw holes to attach the cover. In fact, the cover (I actually found 2 different types left at the house) sits perfectly in the inlet, but the screw holes align where there is no plastic to actually drill into (they're inset from the edge about 2 inches, and the rim of the inlet is only about an inch, so they'd screw into nothing). I also inspected the inlet area and there are no holes in it already from a previous cover, or for a new cover to drill into.
So the question is, how are these normally screwed in? Could it just be they never had a proper drain cover for the pool? Or do they normally drill one on when they install the pool and never did it right the first time? Or are there covers that are used for my kind of inlet that will work and won't move (like a weighted cover)? To make things tougher, we have water in the pool, so I can't take pictures, and any work would need to be done by diving down. All the inspection and testing was done just by going down with goggles, and I don't have a scuba suit or anything to do any higher effort work.