Putting a motherboard in a new case, grounding it question

I built a new case for my computer out of aluminum, when I welded up the sides to the bottom, I didn't have enough weight pressing down so the bottom became warped. The case has a slight bulge in the middle.

Now to my question. I want to place the motherboard into the case, but I realize that if any part of the motherboard is touching the aluminum case, it's going to cause problems and probably fry the mb.

What can I do to keep this from happening? I have motherboard mounts to suspend the mb, but would it be safe to put something underneath the motherboard such as fiberglass or another material that wouldn't conduct? Just in case the motherboard does happen to hit the bulge in the bottom of the case it would hit that instead of the aluminum?

Any suggestions, information, or input would be greatly appreciated. Thanks!

Crossposting is generally discouraged.

Sorry, I didn't mean to crosspost. This topic was specifically limited to my concerns with the motherboard and having in ground and possibly touching something non-conducting and if that would be an acceptable means of mounting it. I figured it would be more suitably placed in the motherboard forum, whereas my other topic addressed other parts I should be concerned about.

But for future reference, I'll try to avoid such similar topics.