I've standardized on making fixtures from IKEA pucks, mostly because of the ease of mounting, ready local availibility, and the somewhat cooler color temperature that I like.
To control the voltage I use a "Meanwell" DC to DC unit from Jameco that will produce a tightly regulated 12v (adjustable, actually) with inputs from 9.2 volts to 18 volts. P/N 175805, 25.2 watt output. Even with significantly fluctuating input voltage the lights
do not dim or flare up.
The usual switches I use are double-pole variants of some commonly used RV switches. Therefore, at the price of more convoluted wiring, the converter is activated right along with the lights and the whole thing is seamless in operation.
Most of these have been in operation for several years, with no hint of trouble.