Hi UAT,
I have a later model en not the 301 panel fitted with the relays anymore, but can give some tips you may find useful.
Pick one of the F’s which are not occupied, very likely they are equally fused. Connect your “+” of the USB’s to the free connection of the chosen “F” on the board and link your plusses through. Connect your “-“ of the usb to the common “-“ rail and you are done!
Seems a simple job to do and I would not propose any additional fusing as it is already fused on the panel