Hello,
So since it’s so short of time and you want it to be cost effective, does your fridge have an ice maker? Why can’t you just drop some ice cubes in there.

sorry I had to but I do have an idea that could work.
Running a line from the fridge wouldn’t be a horrible idea, except, do you have it hooked to an rodi? I would make sure that any water from city pipes do not contain copper in them. Another idea is maybe to consider dry ice. Now you will need to handle this with caution, but in one of the labs I worked in, used a small fryer grate to place the dry ice in. With some math and your water capacity, we can figure out exactly how much dry ice you would need to cool the tank. Definitely need small amounts because too much would freeze your tank. Now I’m the labs when we placed it in the water, it did have levels of outgassing, (you do not want to breath the outgasses or Aka vaper directly, it can cause hypercapnia, but to solve the outgassing we used ventaltion hoods for the larger labs. But, the small ones we just used a large sized normal floor fan and placed it by the window. I would have to sit down and figure the math out, but it’s possible. There is a special container we placed the ice in as well. It did keep our tanks from 68-72. Depending what our experiment was we reduced or added more ice. I haven’t bought dry ice in awhile (that’s a good thing), but it should be fairly cheap.