So to be clear, this is to use when you are able to make it back to gringolandia?
where are you in ticoland? Im very familiar with the country and region, hence my gringo joke..

there are of course purpose built rain covers, but going from your posts, plastic bags in panniers are probably your easiest option.
or pop over to Guapiles and snag some blue banana bunch covers (that's a joke)
aren't you the guy asking about a rear rack on a road bike? If so I don't know how you'd source a streamliner, import duties on bike stuff are high in cr, not to mention no flights.

if you go the bags in pannier route, go with thickest bags you can get, less holes ripping in them chances