I didn't take a picture of it (because I'm an idiot) but last Saturday, at a car show with Tyler, we saw an old Chevy truck with a super soaker bottle for an overflow tank. Not saying that you should go that route, just thought it was cool.
There was also a riced out civic with a rockstar can as their overflow tank. But that's not nearly as cool as the super soaker bottle.
now that I think about it the meth tank I am making could work of course as an overflow and would both clean up the bay the way you want while keeping access to it easy. I started off with a 2 x 4" piece of extruded structural aluminum (1/8" thick wall) that was 25" long and then welded some ends on it. The ends double as the mounts for the radiator and like I said it sit right on top of the radiator.
I think all told I have maybe $50 in parts into it.
And I was a little surprised to calculate that it will hold just under a gallon.
Just an idea, is there anyway you could reconfigure your washer fluid tank into an expansion tank. It's mostly hidden, and easy access. I know I hardly use mine
I actually did come across a thread where someone mentioned that they did this. I don't think it would be too hard. just install a fitting on the bottom and ensure its sealed up tight. My only concern with this is that its part of our state inspection process here that the windshield washer works.