"snug " fit is all you need
you need to go back and read and look at the pics ! page 2
there is a back flange glued into place , which acts as a backing for both panels . then both panels are V noched and skiffed with filler ..
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I think you're getting WAY too in depth. If that's a huge concern for you, how much research did you do before mounting your bumper or lifting your 4runner or any other mods for that matter?