Do NOT put the bottles outside the res. They will simply condensate, evaporate, and increase humidity.
Put 2L frozen bottles INSIDE the res, and you will have to do it every day, perhaps multiple times.
Also, a fan blowing over the res WILL cool the temps. HOWEVER, this is EVAPORATE COOLING, which means
water is now being removed from the res by PLANTS and even more by EVAPORATION. This means pH will DROP
as nutrient concentration in solution INCREASES. You will need to top off with fresh water, perhaps daily, to maintain
steady pH. (Highly dependent on the size of your res in gallons)
Best solution I've found: (And it's best in RDWC/UnderCurrent - Single buckets is pretty impossible to do EFFICIENTLY)
Get a Stainless Steel (NO COPPER, It can leach into solution and poison your plants!) WORT CHILLER, and a 1/10 - 1/4 HP chiller and run cool water through the metal WORT CHILLER coil and set that coil in your res. The steel cooled by the water will then cool your res/nutes.
If you are only doing INDIVIDUAL buckets, the frozen bottle is the best solution, which isnt really saying much since you're then tied to your garden.
Look on craigslist and I bet you can find an aquarium chiller at a decent price.