Biodegradable adhesives
Taking "India" into account, you could surely find jackfruit or mango tree. I would suggest you could use the white sticky latex of jackfruit/ green mango. While eating, I find extreme difficulty in removing the gum like plant excreta from hands,  I am sure it would work as a mantra as far as natural gum is concerned. 

Secondly, in Indian villages you could see loads of mud house decorated with cow dung. Natives throw the just released cow dung to the mud wall even half roasted bricks wall to dry them. So that later on, they could easily take it from there, break it and use it as fuel. As it has a sticky power, I believe it could easily stick with something. If you're joining two halves of coconut shell, use the just released cow dung and symmetrically spread it over the edge. Then simply put the other one over that and let it rest. It will work as a very natural adhesive.