Just a couple of thoughts..
On the
battery one, I found the suction cups do not hold and on a day sailor used the "stud mount" where you affix permanently the plate then use a cotter pin to insert the light when wanting to use it...except for the problem of the
batteries, that arrangement worked fine..
In the onboard battery, I don't have experience in this but others might...would a gel cel motorcycle/jet ski battery
work? (they are small and can be inverted with no acid issues).