What materials should be used to seal a square sump pit
I would install a frame built out of treated lumber to support the lid. Anchor it to the concrete with construction adhesive and concrete screws (Tapcon). It appears that your pit is very shallow, so you may need to build the frame above the pit and surrounding it to gain height for the pump, etc. I'm thinking 2x4s against the walls and 2x10s oposite, flush on top.

Then, buy or build a polycarbonate lid using appropriate grommets for the penetrations. This company offers a great selection of parts (and a ton of good information). If you want to use one of their round covers, put a 3/4" treated plywood deck on your frame and cut a circular hole to match. Screw the lid on with a strip of foam tape underneath.

On a related note, Radon mitigation systems don't rely solely on barriers to Radon gas, per se, but airflow management. By venting underslab air, Radon is diluted and dissipated to a safe location. Simply sealing a foundation is unlikely to be successful. I'm assuming here that a fan or static suction mechanism will be installed.