This is what I tried, and it worked -- so I stuck with the same formula for refills. It is possible that other proportions or combinations would work. In fact, I probably didn't make it up exactly the same every time. Basically this is it:
In a 16-oz spray bottle put:
15 drops cedarwood oil
15 drops bergamot oil (bergaptene free)
5 drops rosemary oil
5 drops tea tree oil
5 drops peppermint oil
5 drops peppermint oil
fill the rest with water
Spray on all exposed skin as well as on clothing. For the face, spray on hands and rub on face avoiding eyes. The only caution is some peoples' skin may be sensitive to one or more of these oils. None who used it on our trip had any adverse reaction (or any mosquito bites!) Also, if you have it on your face and then sweat so the stuff gets in your eyes it might sting. But probably no more than chemical repellent would.