These lightweight glass ballast tubes are open one end for 19 mm or 3/4" ballast slugs.
Make the slugs by filling 3/4" copper pipe with lead shot and epoxy resin, and then cutting it into 5 cm / 2" long pieces.
As used in many 2 m - 3 m slope soarers.
Internal diameter: 20 mm
External diameter: 21.3 mm
Length: 34 cm
Weight: 24 g