Yeah I'm pretty sure it doesnt need a hole drilled in the top sheet. We have tried that trick with filling the hole using a mig and all it does is push the two sheets apart only success i had was when i clamped a solid piece of rod to the back layer but sometimes thats not possible. Will give this a go only bummer is i have to go buy an arc welder capable of 60 Amps but will just try a cheapo bunnings one.