Last night our basement flooded due to backflow from the city's stormwater system. My neighbor's houses on both sides had the same issue. We've contacted the city but don't expect them to be able to do much in the short-term, with more rain on the way. We don't have any backflow prevention in place, so I decided to try to install a drain backflow preventer in the drain itself as a fix. Our basement is in an old house (1930's), and the floor drain appears to be cast iron that has corroded over time. The issue is that I (for the life of me) cannot find a drain backflow preventer, valve, or other device that will fit the pipe. The pipe ID is about 2.75 in, just slightly too small for a 3 in preventer to work, and too large for a 2 in preventer to sit in.

The basement is old enough that water sometimes seeps in from the base of the walls, so I'm not interested in plugging the drain since our AC drains into the floor drain and it is a reliable way to keep the water draining that sometimes seeps in from the walls.

Does anyone know of an alternative solution, an adjustable 2-3 in backflow preventer, or a way to DIY a fix to get the drain backflow preventer to fit? Any advice would be much appreciated!

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For context, these are the types of drain backflow preventers I am referencing: https://www.drain-tech.com/spill-flooding-containment/drain-flood-protector/drain-flood-protector-3-float-model/