Conservationist ask 4 help, fallin out of trees!

Hey so I live in Kenya and I'm working on a conservation project with local wildlife. We build bridges that hang over roadways to allow monkeys to safely cross. Our problem is that we have no safety measures in place when were working on top of some of these trees. Recently we had an accident where some of our guys fell after while up in the air. fortunately no one was injured but we may not be so lucky next time. We don't have a lot of money to spend on equipment or any money for that matter. What were doing id very important and I would appreciate any ideas for cheap safety measures which would allow us to continue working. I understand that this isn't the best way to approach the task at hand, but we don't have many options. Thanks for your support.
Andrew
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Do you have ropes?

At a bare minimum you're going to need rope and harnesses or the equivalent of a harness tied with rope or webbing.
-moss

We can get ropes. Dose a harness made from old fishing nets seem like a good idea? It would of course be thick enough and we could do low level testing but I really know nothing about the subject. Then would the idea be to climb the tree and tie off once up there? One problem is that we are often at the top of the tree so there's nothing above us really to tie. Even if it was below I suppose a short fall is better then a long one. Are guys are good and the trees are generally easy climbs its just when were working up there its easy to slip. The common sense part is covered but I imagine there are some areas we could over look and not being funny I do fear accidental hangings. How important is knowledge of knots? Were pretty good with them but were not professional. Thanks a lot I really appreciate it!

Just Curious
Why would Monkeys need to have places to cross Roads on the trees above? They are usually smarter than crossing roads where they are likely to get hunted down for their meat. Sounds fishy to me!
Are these big roads like highways or just small dirt roads (most likely in Kenya.)