eTrex uses standard serial interface for communication with the PC.
The connecting cable could be built from several (centi)meters of shielded 3 wire cable.
On the computer's side ends with normal 9pin Cannon. The eTrex is plugged using special
connector ... thats the biggest problem ;-]
The connector is pretty atypical and home-made construction with good reliability
is difficult (but not impossible). The original connector consists from
rubber chasis with three spring driven pins, but the construction with flexible copper
reeds could substitute this very good.