I was thinking maybe if i could find an old weight set at a garage sale and pick up some old 25 pound plates. But then i would be carrying a bunch of barbell plates in the back of my jeep.
Indeed; carrying that much extra weight is not interesting. Because of space but also for fuel economy, like said in comment #32 quote : "Use your fire wood, jerry can, cooler, spare tire, or some sort of empty bag to fill with whatever you find. " <-- that in combo with some nail like heavy duty spikes and stuff that u find aroun will do. Using firewood and stuff is oke if there is very low wind. A tarp can put a lot of force on the ropes and anker points so keep that in mind. If wind picks under it it can make a blok of firewood go airborne!...
I think you will be fine with a heavy duty nail stake on gravel. It could feel loose in the grond and then just place some extra weight like a block of wood or rock or anything on top of the stake. Buy enough stakes cause u mind go damage them rather quick in hard packed surfaces.. + in heavy winds you just use extra tie down points to spread the stress wind puts on everything.
Did the same on this hiking trip in Norway (Trolltunga), spike hammered in the a crack in the granite and then a granite block on top of the stake (or the rope just around a heavy block).
Head some heavy wind that night and no problems at all.
(I know its not the same with a tent cause a tarp catches much more wind..)
:)