Minimalist setup for a "travelling bike mechanic"?
I'd be very tempted to make a tool roll. You'll need access to a sewing machine that can handle 2-3 layers of canvas, but even most home machines will do that if you're careful (and buy a canvas needle!) 

You see them mostly today with sets of ring spanners, made of cheap plastic. But in the older days people would generally make them out of canvas, often double layer with oilskin/oiled cotton on the inner layer (before chrome plating meant most tools don't rust very easily).



The extra material at the top of this pic lets you fold the "flap" down to stop tools falling out when it's packed up.

A reasonable sized roll will also let you lay out parts if you need somewhere less dirty than the floor. And it can be roughly rolled up with tools and parts loose inside it if you want to carry it a short distance, or just to make it less inviting to thieves.

Make the first one any old how, because there's a bit of a learning curve in what works for you.



As a secondary thing, I carve my name into tools with an engraver. Those are fairly widely available these days, or most key cutting places will have one. My decent/valuable tools I keep with me at all times, or just don't take to places where I'm worried about losing them.

I also still lose tools at events like this, almost always by lending them to someone who doesn't return them (often innocently, by giving a tool to someone else and saying "it's Mσᶎ's, give it to him when you're finished with it"... and after a few people have passed the tool on it's impossible to trace). I'm in a financial position where I can afford to hit bargain bins for cheap tools, because the ones that go missing are usually adjustable spanners, 15mm or 10mm ring spanners, and 5/6mm allen keys. All of which turn up in bargain bins for under $5. So every 4-10 trips to community events I spend $50 on new cheap tools. Often tools I wouldn't personally use if I had a choice :)



I'm thinking of buying 10 or more of the "S shaped" double ended 5mm/6mm allen keys because I expect I could get usable ones for about $2 each, and I'm willing to spend that to not have to worry about losing them. I expect that I could probably sell them rather than lending them at that price.