How to seal sump pit with removable cover
I am currently doing the same thing; while sealing it for a radon system.  The radon pipe will be through the slab in a separate spot in the basement.  I am using a piece of clear Lexan with rubber foam rope insulation underneath it.

Here is my finished cover.  It works fantastically.  Access hole with removable seal.  Uniseals for the cords and pipe.