As I suspected those catch boxes are for rain water. (Probably angled with an opening at one end.) I too think the screws are to hold them in place. As I stated before, those boxes are not needed. In fact they may cause more problems by blocking too much air flow.
When my S2 was on the road, rain entered through the vents and ruined the paper air filters on the DCOE's.
I later built an air box but rain still fell on the mild steel (painted) exhaust on the other side.
A lot of people don't take their car out in the rain, so perhaps it doesn't matter to them.
As far as I can see, there isn't all that much air flow out of those vents. My data point for this is that a leaf sitting on a vent won't blow off until you get to about 30MPH or something like that.
That said, the BaT ones look to be made of steel with perhaps an opening at the front. I personally wouldn't be drilling holes in the deck to mount them but an S2 already has screws to mount the vent screens, so that'd be the obvious choice.
So, I reckon some lighter and more attractive rain deflectors could do the job adequately. They could be left open on all sides with a small lip to guide the rain water. After all, we're not looking to control Niagara falls.
I must sound terrible pointing out every little fault and I will apologize if it’s a bit much.
All good with me.