I had my doubts at first too!
I have a set of three by five rectangular driving lights mounted on the magnets and have pointed them front, back and sideways at various times, on the rops, fenders, fel brackets and even on implements. I have not had them shake off or to a different position but the lights are fairly light and are mounted closely to the magnets. I have them wired into a lighter plug and have wired several lighter sockets on the tractor. Zip ties keep the wires where I want them.The lights are 55 watts each and throw lots of light. I think my total investment is 30 bucks.
I have been thinking about making a thermos holder with one of the same magnets and a piece of ABS pipe.
Try it -- if it does not work for your lights you will only be out the price of the magnets (about eight bucks) and everthing else will bolt to whatever bolt on brackets you choose----and you might discover the best place to put the lights for permanent mounting.
Almost forgot -- you might want to look at a set of atv holders to mount the weed wacker