Drill a hole through the tube and seatpost and shove a bolt through it. When you want to make it adjustable then extend the hole in the seatpost so it's a rectangle with rounded ends and get some 3d printed pieces of the same shape with the bolt hole at different heights.
Drill a hole through the tube and seatpost and shove a bolt through it. When you want to make it adjustable then extend the hole in the seatpost so it's a rectangle with rounded ends and get some 3d printed pieces of the same shape with the bolt hole at different heights.