168MVC, is the sheet metal supported across the entire 16x16 area, or is your Delrin/UHMW part hollow? If hollow, I'd be a bit concerned with drum-skinning effects of your 18ga. If this is dynamic in any manner, it will loosen your rivets.
If you have to use rivets, I would suggest you keep the ceneters of your holes away from your edges by at least a factor of 5 for the diameter of your rivets. AS the rivet collaspes, it will try to deform your plastic parts. If deformation isn't a concern, then you might be able to mave as close a 3x the diameter of your rivets.
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