Just take the semi-stiff, clear, plastic that comes as a part of so many things that you buy these days and cut it into a rectangle that's about 3 times as wide as you flash head and about 2 1/2 times as high.
Then fold it approximately (but not perfectly) in half so that it's easier to separate the two pieces to swap the gel out while it's on the camera. Next, match it up to your flash head -put the flash head in the middle, then fold the plastic so it fits the width of the flash head perfectly. When folded, it should look like this:
A rubber band on the end of your flash will allow you to tuck the tabbed ends of the flash holder under the rubber band on either side to hold it on your flash like this:
Just make sure the opening on your folded gel holder is pointing up so the gel doesn't fall out. But, this design allows you to swap the gel in and out as needed.