I am going to try and make myself a colored translucent face plate to match the buttons I want, so I have a question.

Are the grooves, pits, valleys and voids on the BACK of the stock faceplate there for a reason ?? ie the clerance is needed to make it fit ?

They're there to make the plastic easier to mold and to use less plastic. The empty spaces could in theory be filled if you wanted them to be.

You need to make sure that the clear lens fits properly into the fascia, though. It doesn't lie on the back of it, it kind of notches into place. Careful examination of the original product will show you what I'm talking about.

I did that once
check it out
But yeah, what Tony said you need the stuff on the back to hold your lens.

Also, a groove at the top allows the unbent lip on the defective sleds to slide into the fascia. If you've got such a sled, take a look at it outside the car, if you can. If you filled that groove in, the defective sled would suddenly become much more so.