accidentally broke passenger side pin/fastener holding down plastic cowling - replace ASAP?

When reassembling after my unsuccessful attempt to remove my windshield tank (can't remove washer fluid tank, trying to replace passenger side HID - PriusChat Forums), I accidentaly broke plastic pin/fastener holding down the plastic cowl on the passenger side, next to the fender. It's in the 2nd picture under step 2 of the Word doc at http://priuschat.com/forums/gen-ii-...r-brace-install-pictures-text.html#post570085.

At first, I wasn't too concerned until I came across the above instructions and efusco's warning about making sure the cowling goes on right. Should I be concerned about my currently missing pin? It rains here quite a bit and I wouldn't want water leaking into the engine compartment.

I originally figured I'd get a replacement pin the next time I'm at a Toyota dealer, but that could be awhile away.... Should I make the trip sooner than later?