How can I fix and cap my undersized sump?
Buy a steel or aluminum sheet metal plate big enough to cover the hole. You could use a silicon bead under the edge if you really need it to be gas tight, but if you do, when you add a new sump pipe remember to also add a hose to allow air to return into the sump when the pump runs.

Thickness of the plate will depend on the hole location and whether you expect anyone or anything heavy to stand on it.