TWIMC...

Assuming the drill needs to be held against the bicycle's bottom bracket
anyway, a non-locking connection turns out to be easy. Lowe's sells 1/2
inch 20 TPI nuts in their specialty drawers (2 for $1.67). That nut fits
on the DeWalt drill spindle. The bicycle bottom bracket tapered spindle
is about 1/2 inch square. So a 1/2 inch drive socket that fits the drill
spindle nut neatly completes the connection.

I might carefully grind a little bit off of the bottom bracket square
tapered spindle and/or maybe the socket drive area so that the 1/2 inch
drive socket slides on a bit further, if I can confidently do that.
Don't want to make it off-center though.










Electric motors being so straightforward, for example a cordless
drill, switching it out for more power should be easy enough.
Especially if I can come up with a reasonably simple and reliable way
to connect the drill spindle to the bike's "bottom bracket" where the
pedal normally would be attached. I've got the other stuff worked out
(I think) and it's under construction. That drill spindle to bike
bottom bracket connection seems the most difficult.

The bottom bracket axle appears to be very hard steel.