DIY running board covers using rubber bed mat

In my case ('40 Chevy Master Deluxe coupe) the bare metal running boards themselves are ribbed. So how do you apply your rubber matting over that without low spots? And yes I would want the same ribbed look when they are redone. (Also, I hesitate on this one based on quality but Rock Auto actually shows they sell the rubber replacements for $245.00 for my car.)
I know this is an old topic but just joined.
Thanks.

 

If your '40 Chevy is like a '39, the rubber mat is bonded to a metal plate that attaches to the running board. They are a smooth surface w/ low ribs that cover up or bridge over the low spots on the running boards.

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