Is your location actually Costa Rica? That sounds like the kind of rain that will soak through a DWR treatment on non-waterproof cloth.

If the budget won't stretch to new Ortlieb, you could look for used Ortlieb on ebay etc. Or Alibaba has plenty of vinyl-constructed panniers, I saw some for US$25. Yes buy once cry once, but that's 1/6th the price of new Ortlieb in USA