Butt soldering wire holder.

thingiverse

This contraption holds wires end-to-end while you solder them. They use magnets to attach themselves to a metal work surface and apply slight clamping pressure. They clamp down on the wire like a clamshell, with the foam applying light pressure. The lever makes it easy to open the lid. Instructions Print two sets. You shouldn't need support. Print the "lever" on edge. Use 1/8" or 3mm rod for the small connectors -- you might need to drill out the holes for a smooth fit. Use 1/4" or 6mm rod for the main hinge. Again, drill the holes if needed. Use two screws to "lock" the bases in position on the hinge rod. Use 1/16" rod or a finish nail to attach the lever to the lid. Glue 4 magnets to each base and 2 to each lid. The indents are sized for 12mm x 1mm round magnets. Glue in a square of low density foam to the inside of the lid. If the foam is too thick or stiff, the lid will not snap down and hold. I used the foam that comes in microchip packaging, but had to test several types before finding the right thickness and density.