Hi Mjenki
There is not really a lot to the construction of a beam drill.
Basically you are looking at a horizontal beam of timber, one end is fixed to a vertical post so that it can pivot up and down like a fulcrum.
On some fancy beam drills it is possible to raise and lower the beam to suit the work and also pivot the arm if the upright beam is round.
One some of the drills I have seen, the under side of the horizontal beam has a metal plate attached along it with a series of blind holes in it to take the end of the brace.
At the other end of the beam to the pivot would be a weight to provide the downward pressure on the brace or maybe a chain/rope for your assistant to haul on.
The fulcrum and weight provides the downward pressure you turn the brace.
The brace would be metal with a square socket at one end for the drill bits and a straight shaft at the other with small rounded tip to fit the holes on the under side of the beam.