A better mouse trap!
I will try to help you out if I don't get overwhelmed. I too have suffered with this problem, and have refused to pay the 1 or 2 dollars each for these cheap stamped steel nuts. I have not replaced all of mine, just the ones at the front of the most forward pan. I have used an aviation nut called a clipnut. I don't have all the fancy photo capability at my finger tips that Mike Tangas has, but here is a simple description that I hope you can follow. A clip nut is captive threaded nut, that is designed to slip over a thin flange (the MB chassis). When another panel is laid over the top (the plastic belly pan), a screw can then be driven in (no access to the back side is required). The standard nut is held captive in a u-shaped spring steel clip. Not only does the spring of the clip keep the nut press against the back side of the stationary surface, the bottom of the u contacts the attaching surface during screw installation to provide the wrenching resistance needed for screw installation. The only thing I don't like about my installation is that the large rectangular whole for the MB spring nut is kind of too large for the clip nut to be totally idiot proof. On installation, I need to carefully position it so it bridges the large gap. Once you get the screw started, the front pan will never fall off.
If any one wants a set of four or five clip nuts, 10-32 Phillips head screws and large diameter washers, please send me an e-mail and I will put them in the mail to you, no charge, unless the volume gets ridiculous! Consider this a late Christmas gift or advance payment for technical help I will need in the future!