I'm still in the middle of the long process of building a track. The track is 100 by 150 so its alot of work just keeping the weeds down---spray, weed whack and repeat! Anyway, about using drums or barrels for filler. I've talked to individuals who have built several tracks over the years and they recommend NOT to use any filler. The only thing that sticks to dirt is dirt. With use the drums will poke through and you'll end up doing some patching. That being said, I have seen tracks that do use that in the construction, so its not unheard of.