15x15 mm stand for square round tubes

Clamp at the table with clamps.

15x15 mm system for square round tubes from DIY store

Inside Dimension = 15.5 * 15.5 mm
Nut Size M5
Nut internally

Easily printable, very stable
0.8 mm shell and 25% filling enough

For aluminum hollow section profile is the weak point.
Tighten the screw only slightly.