orange4boy -
Great Job! If I were you, I would put holes in the bottom in order to let rain water drain out. I noticed that my Saturn's plastic running board is hollow like yours and has "unused holes", so I figured drainage was the reason.
For holding it together, you could also use plastic fasteners like these :
PDQ Automotive Plastic Fasteners Online Store
You can even get some in white :
CarloSW2