Many rod connecting elements can be found on Thingiverse. These are for 6 mm wooden rods. The bed side is slightly flattened, so they print nicely without support. Even the 60 degree roof elements.The connecting rods can have any length. Here they are 12 cm so the kids can make their things on the table. For those who want to make different connectors, an Open SCAD file is attached. For different diameters, use scale in the slicer. Use a 6mm drill to remove eventual inside roughness by hand by clamping the drill. If you use a machine, be careful and drill at low speed..