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For a 2.5 lb ground weight balloon, a radar reflector is likely still too heavy. The lightest weight marine radar reflector [1] I could find is about half a pound.

[1] https://www.westmarine.com/plastimo-tubular-radar-reflector-...

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you can make one out of cardboard and aluminum foil (better yet, aluminized mylar (space blanket) on foam) on the order of a few oz. https://www.instructables.com/Lightweight-Radar-Reflector/

A radar reflector such as that, or this (https://overlookhorizon.com/product/radar-reflector/, which is ~300g) has roughly the same RCS as a small (piper cherokee) to medium (gulfstream) sized aircraft.

That being said, detection isn't everything; primary radar cannot make accurate altitude measurements, only bearing and range. While that's enough to route traffic around, it could be also mistaken for a false return.

Any idea how much that homemade one weighs? 300g for the commercial one is more than half a pound, similar to the one I linked but a different shape.

Marine applications care a lot about durability and pretty much zero about weight, so I wouldn’t expect that to be representative of how light they can be. You could make a radar reflector from a bit of cardboard and aluminum foil.

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