Pic of kind of grommet I need--Where to buy?

This is a follow-up to an earlier post.  I'm looking for a grommet that will allow me to bring 2 wires (joined, about 16 gauge, for an external 3rd brakelight I'm installing on my louvers) through the metal of my hatch. 

It will be visible, but sturdiness and waterproofing are more important than looks.

Here is a grommet I ordered from a marine supply company, but it is too big (they have none smaller).  I've seen a smaller one that would be perfect, but I can't find it for sale anywhere.  The nut at the bottom of the picture screws onto the threads toward the bottom of the picture.  As you tighten it, it clamps the grey rubber piece down on the wires.  Anyone know where I can get a smaller one or something else that would work as well?

Here it is.  You are looking at the top of my car and the top of the hatchback.  The LED brake light strip will go on the louvers, at the top.  I had and am still considering roof-mounting the strip, but fear it might ruin the look of the car.  It is the same strip you see on top of a lot of GM vans and SUV's, long, flat and unobtrusive, but very bright. 

There are 2 holes.  The larger, centered one is for the rear glass sprayer, the smaller one to the left for the grommet (I had intended to use another type and changed my mind). 

Whatever I use needs to be tough.  It will be exposed to sun, weather, carwashes, etc.  Thanks for any suggestions.

A basic 3/8" grommet was what I was going to use when I drilled the hole.  Then I started thinking about how quickly water ruins ZX hatches, and I decided I needed something better.

I can see how your suggestion would make the 3/8" grommet more permanent and waterproof.  That may well be what I do.

I just can't believe that there isn't something better suited to this.  Seems like  running wires through the exterior steel shouldn't be unheard of.

Thanks for the suggestions!  If you have any more advice, please post it!