I came across this yesterday, which I suspect many of my readers will be interested in.
The maker refers to it as “Vietcong” but such weapons were more often associated with the Montagnards. I particularly like the clip used to hold the bolt in place. The trigger mechanism was new to me. It is simply a piece of wood carved into a T shape and dropped through a slot in the stock. This site gives you a better view.
The video below is more detailed, even if the final product is less elegant.
I provide some information about field-made crossbows in my book, although obviously I can only deal briefly with such a rich subject. One of the advantages of the crossbow is that the bolts are considerably easier to fabricate than conventional arrows. Conventional arrows need to deal with the Archer’s Paradox so are more sophisticated devices than you might at first expect. Head weight, flexibility of the shaft and bow power all need to be balanced. A crossbow bolt is a simpler device, but not without its nuances. Have a read of Payne-Gallwey’s “The Crossbow” for the finer points of bow and bolt design.
Another very simple trigger mechanism you can make in the field is used on crossbows from the Congo. A notch is cut for the string and a hole bored down through this. A peg, attached to a lever is positioned so that it will push the string up out of the notch. This can be made out in the photo below. On some examples the trigger lever is formed by simply splitting the stock down its grain for a distance.