[dsraven]

sorry, fat fingers hit the send button mid sentence
the other side that has the screw down part has a copper pepe end cap plug fit over top of the jaw. when welding the holes in the firewall I usually use a step drill and drill them all the same size first so firstly they are actually round and secondly I don't have to custom build a plug for each hole. I made a hole punch that makes different sized slugs and keep the slugs in an old pill bottle. then I use the little C clamp thing on the back side to cover the hole, the copper part will help a little to absorb some heat and also the weld won't stick to it. then on the side I am on I just have to stick the slug in the hole and tack it in place, check the fit, then weld it in. it works well. on spots where the slug wants to fall out a little tape on the back side will hold it for a tack then remove the tape and slip the copper back there for the weld. if you plan to build one get a clamp with as much throat dimension as you can so the magnet is as far away from the weld as possible because magnets upset the weld puddle
for a hole punch I used a piece of thick steel, like a piece of 2 inch squre stock by 2 inches long. I bought some cheap pin punches and then used a drill the same size as the pin punch to go right through the block. I have several sizes drilled into the same block. then I cut a slice into the side of the block so a piece of sheet metal can be slid into the slice and will basically block the hole for the size I want. then use the pin punch in the hole to drive a slug out of the sheet metal.