On Special for about $20 at Princess Auto
URL
http://www.princessauto.com/shop-garage/repair-equipment/tune-up-equipment/8303315-automotive-power-alert

Plugs into bike if you have a cigar lighter installed ,If you have the BMW female socket you can source a plug at the John Deere Tractor parts counter.
Mind the polarity of the plug,by North American convention the center pin is hot (+)
Plugged mine in and it agreed with my FLUKE volt meter.
Velcro mounting would do it.
Doubt it is deluge proof.

Helmut

Thanks Helmut.

Princess Auto..... my best store for cheap stuff.
I hate to visit them. They always have some little gizmos on sale and this is like Costco... You go for a $1 item and get out with $100 of merchandise.