All you need is an old towel, or piece of towelling approx. 18" x 6" (45cm x 15cm), and an ice pack like you would use in a cooler for a picnic.
First you need to cut your towel to size. 18" x 6" should be big enough for most picnic ice packs, but check you have enough room on each side for a seam allowance, and an inch or so overlap at the top. If you're cutting up an old towel try to make use of the neatly edged hem for one of your short edges, it will save on sewing later!
Next, turn over the unfinished short edge and hem it. I made this cover by hand but if I had bothered to get my machine out I would have zig-zagged it to strengthen it and help prevent fraying.
Now you need to pin the fold-over top in place. With the right side facing out, position your sewn hem almost level with the top of the ice pack, then fold the towel hem over and pin in place. Slip the ice pack out through the open side, turn the fabric wrong side out and stitch the seams.
And that's it ~ done! All you need to do is turn your cover right-side out, get your ice-pack from the freezer, and tuck it down under the sheets. The ice-pack will keep you nice and cool, while the towelling cover will soak up any condensation so you don't end up wetting the bed ;)