Use thinner card for your templates.....
I'm not sure what welder you have, but if you're butt welding repair panels on the outside, use 0.6 or at the biggest 0.8 welding wire.
You'll need to keep the power low, don't weld continuously, do an inch here, then go on the other side, to spread the heat evenly or your panel will warp and go wrinkly like a witches tit.
Use a damp rag to cool it as you go, slow n steady wins the race. Try and leave a 0.5 mm ish gap to get some penetration because when you linishing it back you need to leave some meat on the panel.
You could joggle the one panel and do a slight overlap.
I would go on you tube if you're on a steep learning curve, because you can pick up some tips on there. Don't be disheartened if it doesnt go well, you don't learn this stuff over night.
Hope this helps