How should I fill firewall holes

[gigamanx]

Did a little searching and can't seem to find a good answer. I'd like to start filling some of the firewall holes I know I won't need on the cab. I've started marking the ones that can go. What is a good method of getting pieces to fill the holes? For the smaller screw type holes, I tried welding one and just made it bigger by melting the edge of the hole, so that's obviously not the way to do it haha. I don't want to replace the metal with a single sheet since I like the curves and bumps of the original one.

Would love to hear experiences and tips on smoothing things out!