That's also part of the recipe for making silicone molds that will set up more efficiently.
I did not try making sugru that way (yet), but the molds are flexible once set.
Of all my worldly possessions, the one thing that goes where I go is my box of Stuff That Sticks Stuff to Other Stuff, and my Box of Magnets, because they also stick stuff to other stuff, as well as supply a lifetime of general entertainment value. Not even kidding. Most of life's practical failures are a direct result of inability or failure to securely tick stuff to other stuff. The sound of one hand clapping? That, too, could have been entirely avoided. (You are now offishully enlightened, btw. Peace, my brothuhs.)
I suggest, with the bike light, consider making a molded containment for your magnets, but you can use the homemade silicone/cornstarch mix for that. (Silicone is used to make non-slip surfaces, so could be helpful in keeping the light where you want it. Can't tell more without looking.) Don't use 'big' magnets at all. Use several small neodymium mags spaced around your mold instead. You can hit any one magnet at just the right angle and break the field, but it's unlikely any one bump could disconnect them all. The plating is basically just there to keep the magnets together, because they are relatively soft inside - but they are crazy strong!
The knifeholders sold as kitchen equipment at IKEA have some really beautiful neos in them. Those also have to be very strong, because...butcher knives vs. toes. Mounted mine on the fridge instead of a wall, nearly lopped off some digits when the mags slammed onto the fridge. So, that good. Plus, dirt cheap. You could snag those magnets for other purposes.