A strawberry container would work, just something to contain a handful of bioballs to surround the water's exit. You could use a sponge with a hole punched in it for the pipe, except it will silt up at some point and may stop water flow. Bioballs will not retain detritus and cause water stoppage. I just did the exact fix last night where an intake in the sump was putting microbubbles in the display tank. We just laid a large 10" diameter sponge over the pump intake and it stopped the bubbles. However, as I said it will need cleaning frequently.
I mean to contain the water in the lower tank, not by putting anything in your overflow.
William Fisher