As I couldnt buy what I wanted here in NZ, I had my wife make me two small cloth bags (sort of a denim material) about 9 inches long x 5 inches deep with a zip along one seam.
They fold up when empty and I carry one in the car glovebox and one in a camera backpack.
I fill them with whatever is available (sand, even dirt, get washed if the dirt makes them dirty ).
They live in ziplock bags that are big enough to hold them when filled, so when raining, they dont get soggy.
Main use is on the window ledge of the car when photographing out the window of our vehicle.
Interestingly, there was an article on Mansurov's Photography Life on this same subject yesterday.
Cheers