I made a buck converter with two irfz46 fets and it turned 50v at 5 amps into 24v at 10 amps iirc, running on a fixed 50% duty cycle, so thats 97% approximately.
You could probably find a pwm generator board on ebay for 3$ and the rest of the components could be found from scrap electronic crap. If you could find a 5v cellphone charger to supply 5v to the pwm board from a 48v bank you could wouldnt have to build a linear regulator to do it, saving a couple bucks