controlling a DIY spot welder with a momentary push button switch?
Using a switch to directly control a large current like this is extremely hazardous if the switch is not specifically designed for this application — if the current is large enough, it will strike an arc when the switch opens. This will rapidly destroy the switch, and may end up fusing it closed.

A homemade switch like the one you're describing ("wood, copper, and some silicone rubber springs") is likely to be particularly prone to this issue, as it is not specifically designed to extinguish an arc, and its contact surfaces are not designed to carry high currents. Do not attempt to build your own switches for high-voltage, high-current devices.