I think there are some real solutions that won't break the bank.
First look at reducing the heat you equipment generates. Light, filters, powerheads, all make heat. If there is a way to cut the time these run or the number, it will help.
Air circulation in the room so that the heat is moved away from the tank into another room? Even opening lids and canopies will help a lot. A simple box fan blowing across the top will evaporate a lot of water.
Sweating works to cool your body. It works the same for tanks.
A combo of small things will often do it for the short term needs you have. I don't favor using ice as it tends to be hard work and lead to temperature swings which are much harder on fish than a slow rise in temperature.
These are all great solutions but my problem is when I am not here for 3 weeks
it looks like the only solution is a real expensive chiller.