You may or may not have seen those "cooling necktie/hair tie" things that you put into water for a couple minutes and they keep you cool all day? Well, generally they cost upwards of $8 each. But really, they are quite inexpensive and really easy to make.
Just as a reference, I spent $15 on the crystals and $5 on the fabric (most of it I got for free from donations), so for 50+ neckties, we spent about $20
DIY Cooling Hair Tie
Fabric (how much will depend on how many you need to make, but for each necktie you will need 4 1/2" x 44" of fabric)
Scissors or Rotary cutter with mat
Iron & ironing board
Water Absorbing crystals/polymers (I bought a 3lb. bag on Amazon for $15, but you only need a 1/2 teaspoon per tie, so I definitely didn't even use close to a 1/4 of that bag. You can find it at Home Depot in the gardening section).
2. Next, sew the two sides together along the long side. You should have a long inside-out tube. Pull the fabric through to make it right-side-out. (My genius husband did this using a broom stick handle. It made things go much faster.)
*Note: There are many variations on this. You can do three pockets, with one large pocket in the middle. Or you can have one big pocket. Whatever you choose. Just keep in mind that though the little crystals don't look like they take up a lot of space, they will expand 10x their size or more, so in this case, less is probably better.