[GeeTwo]

OK, I'm just going to spitball what seem to be the best ways to make this really hand machinable on a drill press (which is the best tool my team has)..

1) Both the caster box and the gear box are made from large aluminum box extrusions, allowing for drill-press alignment of pilot holes on opposite sides of the shafts. -- and drill the caster axle before cutting!
2) use a chain, not gears, between the two axles in the caster box, allowing a much coarser tolerance for spacing than gears would allow.
3) both bevel gears would be held in place on their shafts with set screws or shaft collars, allowing post-drilling alignment.
4) as for #2, a belt or chain connection between the steering motor and the caster box, probably inside the gearbox.
5) a triple bearing around the main drive shift:
a) one radially supporting the drive shaft in the "gear box"
b) one radially supporting the drive shaft within the caster box
c) a thrust bearing supporting the gear box above the caster box

If I were doing this, I would probably make the gearbox much flatter so it just held the steering belt/chain/gears, or move the drive belt/chain down so that both were inside the gearbox.

Attaching a plate on the "top" of the caster box would be the biggest challenge, requiring welding or a rather good bar-bending brake.