The following plans were first published in Popular Mechanics in 1923.
This parts list was gleaned from the article:
White pine, 24" x 2.5" x approx. 1.25" (I suggest poplar, maple or oak)
Small piece of wood for bridge, approx. 2" x .5" x .125"
Tin pie pan
Pack of 5-string banjo strings
Wood tuning pegs. (I suggest geared tuners if using steel banjo strings)
Copper or brass sheet for bridge. (I suggest brass or something stronger than copper)
Original article:
This illustration shows a homemade banjo that has a shallow tin pan for a body. Although this instrument is not intended to compare with manufactured ones in construction and finish, it has a good tone quality and volume. A homemade banjo of this type is within the means of every amateur musician, as it can be made for less than a dollar.
The instrument consists of a 10 inch shallow tin pan and a white pine neck. The shape and size of the neck are clearly shown in the detail. To attach it to the pan, a rectangular hole is cut in the side of the latter, and the neck is pushed through. It is then fastened securely with 1-1/4 inch wood screws, which, at the same time hold the tailpiece in place.
The tailpiece is made from a piece of sheet copper or brass, bent to the shape indicated; it is slotted for five steel banjo strings. While attaching the neck to the pan, care must be taken not to allow it to touch the bottom, as this would greatly impair the resonance of the body. A bridge, about 1/2 inch his is cut from a piece of wood about 1/8 inch thick and fitted in place.
Pegs can be purchased for a few cents each, or if desired, they can be whittled out of hardwood.