I too, do this part time, but how much time are you really working? I just did a job on-site and "worked" for 8 hours. Really, the work came from prepping, setting up and taking down the site. I sat for probably 4 hours while the food was cooking. I was still tired in the end, but you have to put this into prospective. If you are your own boss, you are not going to make a lot of money. It's about doing what you love and getting paid, albeit little.
As for the flies: did you have a tent with walls setup in your prep area? This would not keep flies out entirely, but it helps a lot. You could also setup a fan pointing out of the entrance to keep flies out. Once food is out for serving, that's when you have to make sure you keep the flies away. Keep lids on the food when people are not being served.
Good luck on whatever you decide to do.
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