This is the bit of "Kiwi" that I refer to above... just bigger scale to to a reel....That has a 1.5AH ci battery... recon a 2.5 lithium would be ideal
Then apply that to a reel....I would pick up one of those 'Icon' 14v single battery lithuim drills at Mitre 10...had a couple those over the yrs di8d better than I expected for the price.
Chop the end of a Chinese socket extension 1/2" file/ grind the chopped end into a rounded triangle to fit the jaws in the chuck.
The socket, since it has a little slop in it acts like a mini universal joint along with the bit of slot that I would have drop over the reel handle nut but grips on the reel handle, so holding the drill id smooth...the down side is occasionally u pull the extension out of the socket , drop the socket. That down side is one walk 20m cause it has rolled, and pick it up.
Mention above using plastic tube/ conduct sort of stuff
Yeah.. a hard socket will eventually wear/ dent the handle....a hard plastic tube would be the way to go
Hell thats damn near a electric reel motor/ driver for under $100 ready to go...