One way that instrument making can be more accessible is making the bowl or sides and back out of a gourd. In banjos, fiddles, and other instruments, this was common practice in African-American music and still is in parts of Africa and elsewhere in the world (see ekonting, ngoni, and other gourd lutes). I imagine a mandolin or even a bouzouki, guitar, European lute, etc. could be made this way, so long as the gourd is big enough and there is an appropriate spike through it where the strings can put their tension without breaking the gourd.
It may be more suited to nylon than steel strings because of the weakness of gourd wood, but if designed properly, this may be a non issue.
The top is typically made either of wood, or with rawhide skin stretched and secured over the top--the latter is what African and African-American instruments tend to use, as well as instruments from other cultures around the world, the Chinese sanxian, Persian barbat, Mediterranean lyre, and others.
This is the process of making an ekonting, which is the ancestor instrument to the American banjo:
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