The '817's power plug is available at Radio Shack - part number 274-1532 4.0x1.7mm coaxial, tip positive. I would use 5A inline fuses on both the positive and negative leads.
W4RT sells a replacement, non-fused cable for less then ten bucks that you could use: http://www.w4rt.com/
In the June, 2005 issue of QST magazine there was an article by Phil Salas AD5X, entitled “Input Voltage Conditioner – and More – for the FT-817″. It details a project to add reverse voltage, over voltage, and over current protection, a more convenient power connector (Anderson PowerPoles) and an external voltage regulator to take the much of the heat dissipation from the inside of the rig to the outside. For over five years N1BQ offered the parts as a kit. Not sure if he still is.