hybrid RBA with side magnetic thumb button, and designed to use a self modulating quick disconnect power supply that utilizes super-capacitors instead of batteries. self modulates to any coil for 3.7 volts @ 6.1 watts. you could put all the electronics in the power supply, that way people could could buy others if you make variations later.
but with capacitors the charge time is cut to seconds, then you would need chargers car/home.
be sure to use transformers, cause induction is beautiful. just make sure to implement adequate RF shielding for things like radios, wifi, cellphones, etc...
i almost forgot, machine in a hole, or a loop for a lanyard, i hate loosing stuff. ;P
extended:
the PSU would need 2 charging leads protected by diodes, and 2 discharge leads leading to a board, the board should have a chip to detect the ohms of the coil and adjust pulse width modulation as needed, it would have to be smoothed out by another smaller capacitor.
the charging leads could be recessed ing the casing, and the discharge leads could be on the side, i would implement a magnet in the tip for ejection, both from the unit, as well as the charger, as when the capacitor is full it should automatically break the connection with the charger, for safety reasons and longevity.
the whole thing should be water tight, again for safety, and maybe an auto shut down when detecting a chaotic variable resistance (i.e. water.)
the diodes on the charging leads should protect against discharge.
when testing for safety, think of it submerged in someones pocket waist deep in the most conductive liquid you can think of... safety first.
don't make it out of metal, think more pelican case style plastic, except the liquid chambers, stainless steal, durable glass, durable ceramics...
let me know if you want more.