I watched your stream for a few minutes last night when you were trying to place the manhole cover. I tried to set this as well because it would be cool to have a sewer system for water to flow through eventually.
Anyways, I've been having some issues with ladders and holes for a while now. It seems a 1x1 hole is a little too small but a 2x2 is too large. I did place a manhole cover over blocks last night then broke away the 4 blocks from below and replaced them with 4 curved ramp blocks on their sides which made a perfect circle that the manhole covered just nice. However, the ladder is placed a little too recessed into the circular vertical tunnel I made.