https://www.amazon.com/dp/B0FHQK73W1

Eventually settled on the chinese plexi tubes.  They were 2.75" inner diameter & 36" long, slightly longer than the longest bat & a bee's dick wider, as expected.  There was no way to separate the bats from the sides.  Any o-ring around the fat end would slide up & down.  

Stuffed the fat ends with paper to keep the bats from sliding up & down.  Additional stability might be gained by stuffing paper around the handles.