The factory 5lb counterweight included with the GTI is to heavy to properly balance small payloads. This model, filled with small rocks, weighs in at 647g your mileage may vary.
Required materials.
4x M4x25 socket cap screw (mine have a 7mm diameter head)
1x M8 nut (12.8mm flat to flat OD. Should be the standard size YMMV)
1x M8 bolt/screw at least 40mm long.
M8 nut is installed after printing using the “screw pulling technique”
I would consider putting a bit of hot glue or some kind of rubber pad at the end of your bolt, otherwise the bolt will likely scratch the counterweight rod.
The author marked this model as their own original creation.